It is possible, but unless it is an involuntarily change there would need to be normal x1 rate availability on the flights you want to change to. And that availability needs to be for all sectors on the ticket.

Sometimes it works to make a voluntary change online, others you will have to call or use live chat. If there have already been changes to the ticket or it is an involuntary change (eg. cancellation) you will have to call or use live chat. There may or may not be small adjustments to the fees to pay, perhaps the taxes or exchange rate may have changed. If you don't have a membership tier that allows free changes, you will also have to pay the relevant change fee.

As a data point, a couole of weeks ago I cancelled an award ticket using the cancel option in MMB. The avios were in my account 3 days later, the cash took another 2 days after that.