This job may or may not be bigger than it seems.

You definitely need to break the concrete, excavate to undisturbed soil for new pads, criss cross them with #6 rebar and place the concrete; then put a steel beam across the columns, best to be welded in place.

Provide for adequate temporary support before you remove the column!

For sizing and pad specs, I would suggest a professional engineer would be ideal; your local building department might also be able to properly advise you. Typically, you would need a permit, as this is structural work.

If I didn't do it myself, I'd probably get a labourer for the digging; a carpenter/handy guy for framing the pads, locating the posts and beam and mixing and placing the concrete; and a welder.

The unforseen: you hit a sewage line in your floor, meaning you have to relocate the pad(s), or an immovable boulder or bedrock, requiring a rethink of the support. If you're doing this in a well-built newer home it may be quite a demo job breaking high mpa concrete--up to 5" thick and cutting any underlying mesh and removing 4-6" of gravel.

"We are not hiring a GC for the project. What sort of tradesperson do I call?"

You need a GC.
You are also likely to need an engineer to approve the changes.
A beam to support another beam (what I think you are describing) will also need an engineers stamp.