Why would you want to increase the tr_trnbr?
MFG/PRO sets this using a sequence stored in the database. You shouldn't really change tr_trnbr as it is a unique value for that transaction.
The code that QAD uses to set the transaction number is:
{mfrnseq.i tr_hist tr_hist.tr_trnbr tr_sq01}
which is an include file that takes in a table, a field and a sequence (in this case tr_hist, tr_trnbr and tr_sq01) and calculates the next-value with some rudimentary error-trapping.
If you want to create new transactions, I'd use something like that to generate the tr_trnbr. If you already have old transactions then you shouldn't change tr_trnbr.