If its the ING Orange then as I understand it the interest is calculated on a daily (overnight) basis. So changing the account status should have a minimal effect on any interest you gain.
Alternatively, if you have a variation (like a time deposit account) of the standard Orange account, open up a new joint account and at the end of the interest period transfer the funds into the new joint account.
However, you should really speak to the bank as opposed to taking financial advice, that could potentially cost you a fair bit of money, from a bunch of random posters (including myself) on an internet forum.
JF