UnCategory Mass Change

Yes, as long as they are not split transactions. Let's say you have a bunch of uncategorized transactions at McDonalds. Go to the account register where those transactions are located (or if they are in multiple accounts use the All Transactions register). Type "McDonalds" into the search window then click on the Category header to sort the transactions by Category - this will get all the uncategorized McDonalds transactions lumped together. Select them all then right-click on them & select Edit Transaction. Type the new category in the Category field, make sure there's a check mark next to it, then click OK and the new Category will be applied to all the selected transactions.

If they are split transactions, then you're going to have to go through them one at a time & change them.