A lot of very good advice given here Steve,
To reiterate some earlier responses, if you already at the straight in MDA and NOT visual (with reqd Viz and clear of cloud) you must carry out the MA Proc at the MAP or before.
IF on the other hand you are visual at the straight in MDA with the reqd CIRCLING viz , IF you decide to now circle, say to land the opposite direction you must climb back to the circling MDA for your Category and have the reqd CIRCLING Viz, maintain clear of cloud and visual contact with the landing runway and stay within the circling area.
ONLY the circling MDA will give the reqd obstacle clnce for each category as specified in the AIP.
As others have rightly mentioned a straight in appch is much safer than a circling one ESPECIALLY if conditions are at or close to minimums and even more so at night or in mountainous terrain areas when there are high obstacles/terrain in or close to the circling area.
Many airlines and other commercial operators around the world have restrictions and extra conditions on circling approaches if they are allowed at all at certain locations even when they are published.