Ash is correct in that you really need a second BES, but this can be done quite cost-effectively if you need too as their are some great deals around from various service providers (including ourselves uk based so drop me a pm!!)
The 'kludgy solution' would be to use mail forwarding as you've suggested...lets call you current company oldco and the new company newco. You could setup user accounts on the oldco's infrastructure and then forward email from
. Then give john smith a oldco BB on the oldco BES and he'll receive all his email.
The downside of that is that it's not straigthforward (if at all possible) to get emails to be sent from the BB to look like they are coming from Newco, rather than oldco which could be difficult. This can be overcome with BIS which allows you to set the "reply too address" but this would be unmanigable for more than 5 or so users.
Do you know if there are plans for the companies to merge their infrastructure? - in which case the parallel running of email addresses could be more acceptable
if not then bite the bullet and run with a 4.1 BES in the newco infrastructure.