Probably. My company did the same thing and the power was out for a few minutes.

Yeah but it's preferable to when the cable blows at the end of it's useful life. We had some that was in need of replacement. Three or four times the 13KV line to the transformer between my house and my neighbors blew. They'd drive down my driveway, fire up a generator and back feed the cable with a pulse and wander down the street listening for the arc... Then they'd spend a couple of hours digging up the thing, replacing the bad section, and putting the dirt and grass back.

I've got the bad section on one of the failures sitting on my workbench somewhere. Pretty clean hole arced through almost a 1/2" of dielectric.

So what I am asking if everytime a cable is disconnected and the new cable get connected to the transformer, will I will see a momentary blink? Even if its down the block (NOT the connections on the transformer that feeds my home).

I assumed I would get a blink when they work on my transformer but fear I will see multiple blinks for each new connection at other transformers in area that will reak havok on my electronics multiple/many time during the upgrade.

Power will likely go out. Not just blink.
Depends on how the subdivision is wired.

They should have notified you of the date and time
of the outage, along with the duration.
If they haven't, give them a call.

Unfortunetly no other info from the POCO is available at this time.

Can anyone else speculate on how this process of replacing the underground cable from transformer to transformer might work and how I may or may not be effected by power interuptions?

If the POCO has not notified you, or your neighbors,
then your power will not be affected. I don't
understand why you cannot contact the POCO.
Let's pretend that your power went out completely,
what would you do, post to a forum?

I'm with lbpod on this one. I'm not sure what kind of info you are looking for.
If your power does happen to go out, reset your clocks and move on with your life. :)

If they are simply replacing distribution lines the power vary well may be out for minutes to longer as they connect new lines to the old transformer primaries.

The length of time can be highy variable depending on how the distribution lines are installed initially.

If cable duct with adequate spare space was installed (a rarity except in cities usually) it would be relatively brief.

If they are forced to ditch in new distribution lines (instead of pulling them into existing cable duct) it could be longer. More work has to be done at each transformer than in a typical duct installation.

They have done this in my neighborhood.

They opened a trench and lay the new cable.

They shut off the power to attach the new cable.

They did NOT notify us (me, at least) when this happened. Power was off for several minutes.

If you have a situation where turning off the power would be harmful (medical equipment maybe), you should call the POCO and let them know about it.

"They opened a trench and lay the new cable. "

Think how much money they saved over putting in even small cable duct!

They used a DitchWitch or something similar.

From what I could tell, it seemed that the subdivision was wired using Delta transformer configurations with 2 of the 3 terminals in use. The change was to add the new 3rd cable. But it looked like they also put cable duct down as well.

"They used a DitchWitch or something similar. "

Pretty large ditch witch to meet the burial rules for even 240 v service lines (they are not covered by the NEC, but have a separate 'code').

They might not be as deep as you think. Many places have a minimum cover of 30" of dirt over primary or secondary cables. This is for direct buried or cable in duct. Direct bury is foolish in my opinion but many utilities to this day still do it. I would much rather pull out the faulted cable and pull in new and terminate it rather than search dig and splice.

Those drive in Mailbox spikes that go on the end of a 4x4 post are long enough to reach almost anything in the ground. Call before you dig or drive anything in the ground.

Underground is preferred by those who do not like their visual faculties offended. And buried does offer some storm protection- for the buried portion. With lines on poles, changes in the system can often be done with no interruption of service. Temporary jumpers are installed providing two paths for the voltage while the new conductor is installed to replace the old. Not easy to do with the buried installation.

30 inches requires a very large 'ditch witch' type unit.

The common ones I see barely go 18 inches.

"Temporary jumpers are installed providing two paths for the voltage while the new conductor is installed to replace the old. "

Temporary jumpers are used to carry the whole load if required.

the new overhead lines are installed and connected on the load side.
the feed end is then connected, and the old disconnected and removed.

120 V 'bare hand' work is not nearly as dangerous as 7.2 kV (and higher) feeder 'bare hand' work.

Just for the sake of conversation, they are using directional drilling to bore what I think would be conduit first and then the new wire pulled thru?

They appear ready to begin the project as the large drills and spools of wire/conduit have arrived in the neighborhood.

About 10 years ago they used similar drills to pull new telephone fiber optic cable... I assume these high voltage lines will be a much bigger undertaking.

High voltage lines just need larger conduit.

Some places forced all new POCO wiring to be underground many years ago.
No more poles and overhead lines allowed.

It does crate some comedy when a new house in an old neighborhood is the only one with an underground feed.

The poles are still there, carrying both distribution line (high voltage) and service lines to all the other houses, but one house will have an underground feed.
It is run down the side of the nearest pole and then to the house.

Grandmum, might I suggest that when there are crews
at the work site, you politely ask them about any outages that you may experience during the project And, just out of
curiosity, ask them why you weren't notified, if it
is going to be of a lengthy duration.

"why you weren't notified"

Usually there is no requirement to notify users of disruptions for maintenance.