If your chain is long enough to wrap around the Big-Big (
that is, the 50t Chainring + 32t Cog ... ) AND
if your particular Di2 rear derailleur can accommodate the 32t Cog & the extra couple of inches of chain
then you should be
good-to-go ...
If you are sure that you will never shift the chain onto both the 50t Chainring AND 32t Cog at the same time, then you can probably use your existing chain as-is.