I don't think there will be any issues with ampr-ripd, amprd or the mikrotik script:
- ampr-ripd works with both gateways at the same time and doesn't care about the default route. It is the surrounding scripts that must be updated.
- amprd (my favorite) will adapt automatically to the gateway that is broadcasted as the gateway for 44.0.0.1 (since the first release actually).
- the mikrotik needs only a manual tunnel endpoint update if not already done.
I am more concerned about jnos gateways and some handcrafted router setups....
About that old gateway traffic: remember that all replies from the internet still return via the old gw, since that is the official 44.0.0.0/8 route, and commercial ISPs and core routers don't use connection tracking of any kind.
If the AS BGP announcement of the new 44.0.0.0/8 will propagate, this will cease.