When I had a similar question and found it wasn't the kind of thing easy to 'google' on the web, I called the biggest kitchen and bath showroom place in my area and asked for residential bath sales. The salesperson was very helpful and knew everything about bathtubs. I called Ferguson and I know they have a showroom in Scottsdale. Years ago I went there in person -- I had to make an appointment -- and found it was a good place to get started in kitchen (and bath)fixture ideas as everything is out on display. I recommend a place like that for you to ask your question on the phone.

I asked an experienced remodeler who has done work in the area, and his experience was that the builder always left enough room for the "upgrade" wide tub to be installed without any more concrete work.

That makes sense to me.

If you have 36" available, I don't see why a larger tub would be difficult. You have the space available. Corner bath tubs take a lot of space, so I don't know what your length is you didn't say. Our tub is 42" wide which we went from a 36" and I don't think there was much difference. The drain had to be moved, but it didn't cost us much to do that. Most any bath tub you put in you will have to move the drain, unless you adding the exact size again. We bought our bath tub online and we saved a lot of money. The link is below. I'd call and ask someone who sells bath tubs how difficult the changes would be and what they would suggest.

Here is a link that might be useful: The Bath Tub outlet Store

I think it depends on the type of tub you have now and what you want to install. I have a 30" cast iron tub and had wanted to replace it with a 32" cast iron that had deeper depth to overflow. After talking to 2 people, we've decided to stick with our current tub since the demo and re-install alone will cost more than the new tub.