Yes, you can change from one client to another whenever you want. The clients just control your working copy and the interaction between your working copy and the repository. The metadata inside the working copies used by the different clients is identical.
As of Subversion 1.4, the metadata format inside the working copy has changed. This means for you that once you accessed your working copy with a Subversion client based on Subversion 1.4 and higher, you can not use it again with an older client. You will get an error message telling you that the client is too old.