Replacement Leisure battery - AGM?

This is probably a dumb question (and I have searched through the archive to find an answer) but can I just change my lead acid leaisure battery with an AGM lead acid battery? As I understand it I still have the original charging from alternator system from when the van was originally converted from new. I have an expensive Victron 15Amp charger which I am sure can cope with different types of battery when on EHU. Any recommendations for particular brand of battery?

You would not be changing the chemistry so the charging should be very similar if not identical.
Refer to the manufacturer's specifications for the battery you are considering and compare with the settings you can make on that Victron charger, I'd be fairly sure the Victron can be set to an appropriate regime. Victron products are well engineered and reliable.
Now if you were wanting to change to a LiFePO4 battery then that's a quite different story, depending on the age of the Victron it may not be suitably configurable.
That applies only on shore power, of course so you have no control over how the alternator handles the charging of the leisure battery so any chemistry change from lead-acid would need a considerable rework.
AGM is only a slight modification of the old style lead-acid battery, provided your alternator doesn't put out excessive voltage you'll be fine.
I have no idea what brands are available in your part of the world, hopefully someone local can chip in with experiences of the quality of your local brands, batteries seem to be a bit of hit-and-miss, it isn't always the most expensive one that ends up being the best quality. For small(ish) batteries used in electronic applications I've always used Century Yuasa as they seem to be some of the best here.

(As an aside I have a problem with our camper because the alternator puts out about 15.5 volts, I suspect someone has put a diode in series with the alternator sensing line somewhere, but I can't even reach the back of the alternator to measure at the connector to prove it and nothing is visually apparent in the small amount of the wiring I can actually see.)