so managed to convert something like 500000 posts and 20000 threads yesterday and it seemed to be functioning ok for a couple of hours before it broke and started getting db errors, I actually think this is mostly due to godaddy's shared server hosting and the size of amkon's db which is well over 1gb now.
I'm going to look into moving hosts and possibly a dedicated server so eventually we can have everything back. this will probably take some time and obviously don't want to be spending much more than I am already for monthly hosting costs.
so for the time being just going to leave it well enough alone, I think screammmms idea might actually be the best once I move hosts and that is set up the old database as a sub domain, much like an archive but readable only, then if there are any threads people would like to revive we can move them over one at a time.
let me know if this is ok with everyone and i'll start planning accordingly.