"So I'm wondering whether I can still change that to the correct Large Letter rate after the item has been bought."

This is what happens when Ebay ambushes seller listings with SD. By 'upgrading' the postage from large letter to parcel they were going to charge the buyer more, but since you've listed as freepost the postage will be deducted from your sale proceeds instead. I hope they're still above zero?

The good news is, yes, you can buy large letter tracked postage and use that instead. The bad news is you'll have to join the ranks submitting refund claims and waiting for your unused SD label to be refunded. That's if it is, it may prove tricky.

It may be better (whenever you get the chance) to split out postage to get buyers to pay? Then they won't have extra buyer fees charged on top and you won't need to worry about re-pricing everything when the SD rates change.

I'm not sure what happens in the case of express postage but splitting it out so buyers pay would probably save a bit of stress and worry there too?