Emma, we exchanged our UK licences in Portugal last year, and I'm happy to report that neither of us had to sit a test, which is a relief for everyone on Portuguese roads. It was an exchange, not a new licence, but it wasn't quick. We needed a medical certificate from a local doctor, translations of a few documents, and an appointment that took weeks to get. In the meantime we could keep driving on the old licences, but only for a limited period, so we started early.
My advice is to check whether your country has an agreement for straight exchange, because that decides everything. And do it before the deadline, not after, because if you miss it the rules can be very different.