The biggest issue I’ve seen is hastiness - if you take the time to learn each step well and can keep yourself from rushing, then it can be fine.

It’s the ones that try to do it over the weekend because their wife is yelling at them about it that get into trouble.

Is this your only shower? I've done a DIY shower and it took forever, so be prepared for it to take longer than expected.

I just removed an old walkin shower with a tub/shower combo. In answer to your question, yes its possible. That being said you should plan on it taking a while to complete if you need to change any walls or plumbing and thats if you don't run into any strange issues. An extra person is very helpful to help get things into place. If you have a lot of room that might not matter.

The hardest part is lining up the new drain. You may have to trim out some drywall to angle the new tub in place, but overall the process isn't terrible. You'll want to wrap, cardboard and do whatever else you can to not scratch the fiberglass while you're doing the finishing.

Get a gym membership for shower usage just in case. There are many things that can balloon the project, father in law and I did his over a couple weeks, I wouldn't put a "solid weekend"timeline on things. If it's your only bathroom, be prepared for alternative showers, but it is definitely something a layman should be able to do.

As long as it doesn't end up like this: https://youtu.be/lfyaftYvFGg

Sounds like a can of worms. A soft spot in the floor probably means the subfloor needs to be replaced, and if the floor joist is also rotted, then you're really up a creek. I say just sell the house and move to an apartment.

If you aren't sure, don't do it. Chances are, the project will be harder than you're imagining, unless you are starting with a good idea of exactly what it entails.