We have a couple of individual class pages already (www.sheridan88.com & www.sheridan89.com) and someone has just started a multi-class page for the sake of inter-communication (www.shs87-89.com).

The names from 3 classes have already been added to the multi-class site, but no other info. If we uploaded the current spreadsheet from one particular class into the multi-class, would it replace everything or only update the profiles for the names on the spreadsheet? (I.e., if I upload the 88 classmate spreadsheet to the multi-class site, will it affect in any way the '87 or '89 profiles?

First, you can't actually do it quite how you're describing. If you've already added all the Classmates, any Classmate in the next upload is going to be ignored by the parser completely since they already exist as a Classmate in your new multi site.

So what you want to do is actually delete the Classmates' names from the multi site first. And THEN import those other spreadsheets. Also, the answer to your question is no -- doing this will in no way harm any other records or years in the system. Go for it.

Note: You have a cool web site. I like the Blog area. Neat idea.