If you access Classic during this two-week period (using your normal username and password), you will continue to be able to do so after the two weeks are over. If, however, you do not, you will miss out on the chance to do so, so give it a try.

There are 15 members' worlds, one of which will be left as being only accessible to those veteran Classic players who have stuck with it all the way through.
You can access RSC here: RUNESCAPE CLASSIC