" By simply accessing any cached pages it is my guess that we change them."

I don't think accessing the cache does anything. I think the problems are:

1. When the database is restored from an earlier backup, the number for the next thread will be as per the backup. This means that numbers for missing threads will get reused. Assuming the "threadid" is some sort of auto-increment column, the database may well provide a means to change where the next number starts so a gap could be left for the missing threads but I'd guess that Joe would not have this facility.

2. When a thread that predates the restore point has lost later posts. If a new post is added, google will see that version as the up to date version when it next comes to cache the page. I don't see a "restore from cache" solution to this.

I think...

Whatever, I think DaveRo is right that any attempt to restore from archives and caches is going to be a bit hit and miss.