If I use a densifier now does that prevent me from doing a different floor treatment later? I'm doing the densifier now because I won't have enough time to do an epoxy or similar type of job before I move into my new-to-me house. Depending on how well the densifier works I may want to do some kind of top coat later (urethane maybe?). Will that be an issue?
Doing a densifier now does not generally prevent you from doing something else later, but the type of densifier you do and how you do it will have different effects on different systems.
For example, if you just mop on a standard densifier, generally you are going to have the same prep as you would have had if you had done nothing at all.
If you do a densifier with a stain guard or a grind and polish multi layer system, it is going to be important that you grind the surface before doing most other systems -- and some would argue that you should do that anyway on all systems.
The most important thing you can do. Get the full tech sheets from whatever you do now and put them in a file. If you go to do another system later on, send them those tech sheets, let them know that is what you did to your floor and ask them how you should proceed.
As a side note, there are a ton of different densifiers and they work in different ways. You have true straight densifiers (which come in a few varieties). You have some that have a sealer in them and a handful that provide a stain guard.