First, bear in mind that all the "legacy" software products you are clinging to are discontinued and have no formal support. If something goes wrong (see Murphy's Law ) you are very much on your own.

That said, they only thing that I can see that you would lose going exclusively to the Adagio products you list is the Purchase order function in ACCPAC Plus IC 6.5.

I am assuming that you opted (IMHO, foolishly) not to subscribe to the upgrades plans for Adagio since the version numbers you list a several versions back of current. They too will soon find limited support.

Using dated software for a transition period is reasonable. But you, and everyone elso in the organization, needs to be clear that this will end, and you will be using the new, modern, supported products -- or pencil and paper.