Do I HAVE to upgrade to the new wallet?
No, but by now the chain has forked between the Millionairecoin
and Millenniumcoin, as the former does not accept Tor nodes and
the latter does not accept connections from non-Tor nodes.
The old MIL remains functional, but is no longer supported,
and I understand that Poloniex is switching too to the new wallet
if they haven't already.
If you get Millenniumcoin and sync the wallet you'll get the same
blockchain back so you aren't losing anything