I have over a years worth of text messages on this phone and there's some I don't want to completely delete.

Firstly, if I update the phone would every text message / photo / video delete?

Also is there a way to manually move my text messages onto my PC either with a software on my pc or anything else?

if you back up "Communications and contacts" on Backup app, i am sure you wont loose, you can also install GlogArchive app which back ups SMS and Call Logs on an archive file system. you ca then view you messages (and IM's) through glogarchive.

As gabby said, use the backup feature in the menu, i tick everything but communications and contacts does a backup of your calendar entries, SMS (sent and received) and contacts. I had one instance though where the newest backup didn't restore texts so i had to restore the previous one, so to be safe i'd say take at least 2 backups.

I take regular backups and then use mass storage mode to transfer them to my laptop, should the phone break or get lost or stolen, i can just get a new one and it'll be exactly how this one is

if you reflash the phone, it wipes the internal memory. but you can upgrade it OTA (meaning by downloading the updates through application manager, not connecting to your PC and flashing it) and it won't erase everything.
but yeah, do a backup first, in case of problems.
you can also use the latest pc suite or ovi suite to sync contacts & messages to the computer or something, i've not tried it yet though.