[Pizzle]

What it's doing, when the memory is low, is preventing other things, like the address book, calendar, and, well, operating system, from having to purge things considered more important.

There really is no way to merge the old messages with new, sorry. For what it's worth, if they were that important, how come you didn't back the handheld up more recently or save those important texts in your saved messages folder?

Your only option to protect all your data is to do consistent backups. Or delete things on your handheld that don't need to be there. Remove the sample video, extra language inputs that you don't need, etc. There are many things that you can do to prevent the problem from happening again.