Migrate process instances


We are in a process of migrating from 7.1 to 8, using side-by-side approach. But still we will need to migrate some process instances, like the way that is done by install-over migration.
For that to happen I intend to use direct DB inserts in WM8 process tables, using WM7 db as source and thus effectively to copy process instance data only for specific process instance IDs.
My question is - first of all, is that really possible, i.e. will WM8 PE be able after migration to use these wm7 process instances, correlate docs to them, etc.?
And second - do you have a list of tables, which has to be changed during that procedure?

I realise that approach is not completely fine from design point of view, but we really need to close down wm7 server shortly after migrate, but on the other side our processes may take a long time to finish.
Any other ideas how to achieve this are also welcome?

Basically the above approach did work, but it appears JOIN condition handling in WM8 is completely different from the one in WM7.
There is a new DB table (PRTSTEPQUEUE) introduced in WM8, which has to substitute WM7 ones PRTQUEUE and PRTQUEUECONTENT. Unfortunately the latter two are neither deleted from WM8, not a migration DB scripts exists to copy their content to PRTSTEPQUEUE.
Any idea about that, how does this work in case of install-over WM8 Upgrade?
I managed to insert necessary records in that table, but mainly based on assumptions and try-and-fix attempts.
Any feedback here will be highly appreciated.