I think what you want to know really is "Is your presentation layer independent of middle layer".
Answer is: it depends on how well you have designed your UI layer. Spring is based on Dependency Injection pattern and you can very well move from struts to spring without much hassle. But if you want to move from conventional MVC framework of pre-struts era to struts framework for example, then you can not that easily and you may end up rewriting a lot of code.
love your job and not your company;
...because you never know when your company will stop loving you!