Question

If I upgrade to 9.5 and then find out that the team that I work with can't open the project, can I revert back to 9.4?

Answer 1

A given project should only be restricted to 9.5 if that projects opts into whitespace and invisible character support. Once that happens, that specific project can never* be opened with a previous version of Paratext again. 

Just using 9.5 on your machine should not change any projects to require 9.5.

* It could theoretically be undone, but the process could only be done with support staff leading you through a fairly complicated procedure.