Code prohibits decreasing the size of a line in the direction of flow. (Common sense should explain why).

If you pull the tub and replace it with a shower you will be required to make the entire run of the drain line with 2" until it connects to a larger line.

Under the IRC a 1-1/2" line requires a vent within 6' of the trap, while under the UPC they only allow 3'6" from trap weir to vent opening.

For a 2" line the IRC allows 8' while the UPC only allows 5', however according to your profile you are in Canada and I do not have the Canadian Plumbing code so you will have to find out what your local code requires. Generally you can call the code inspection office, any local plumber or perhaps the reference desk at your local library to find out what your requirements are.

All vents must rise vertically until they reach an elevation at least 6" higher than the flood level rim of the highest fixture served by that vent.

afaik. each province has its own code, and they all allow 1.5" drains. Everyone says so, but I've never seen it in writing. Check with a Master Plumber. Any Master Plumber. They are professionals who do not lie. They know 100% for certain what is required and they tell you the truth, when it concerns their expertise; also you do not have to hire them first before they answer questions about their area of expertise. Each Master Plumber will give you the same answer as the previous one and the next one.

Ditto, do not decrease any "drain line in the direction of flow." The trap size has to be the same as the drain size. Many people conclude that a 2" trap can be reduced to a 1.5." pipe after the trap. Use a 1.5" trap if the drain line is to going to be 1.5" diameter. This is very serious, not to be disregarded. Ask a Master Plumber if you need to hear it again.

You may wonder why you see on the internet that showers need 2" ...
I feel that this forum is the right place for this discussion.

It find the 2" diameter requirement for showers to be overkill.

I'm surprised that after many years of searching, I've never seen an internet comment from an experienced Master Plumber telling me how Code changed over the years.

I'm 100% certain that a 1.5" drain line used to be Code everywhere, for showers. I'd like to know what discussion went into voting for the new size, 2" diameter.

(?)
Here is a link showing 1.5" is OK in NY.
http://publicecodes.citation.com/st/ny/st/b900v07/st_ny_st_b900v07_7_sec009.htm

Code has changed over the years and will continue to change in the future.
I'd like to think I'm helping to air out the issues which leads later to clearing up confusion about 2" shower drains.

Imagine all the people who have been forced to do a lot of extra work for no reason !! When a 1.5" drain already plumbed could have been used, they ripped it all out and put a 2" drain in.

I'm not a plumber. Since this is a discussion forum, I'm posting this here as part of a *discussion*; everything I have written here concurs with the previous post from an expert who knows, and who shares, precisely what is code today. Code has changed over the years and will continue to change in the future.