It may be a more complex change than you anticipate: I hope not. In any case, AGMs will likely be a far more efficient and effective battery for any boat that is off the grid with some degree of regularity. To start, the voltage level you stated is for a specific battery temperature and batteries vary widely in temperature and the optimal voltage level also changes with temperature making a battery voltage goal a moving target. (AGMs and Gels are prone to damage if charging voltage is not well-managed.)
A few considerations:
Rewiring is probable wise when shifting to AGMs.
The simple answer is that an alternator can easily charge AGMs if it uses an external regulator of the “smart” variety that allows charging profiles to fit the AGMs requirements.
AGMs are not all built alike, so check the manufacturer’s recommendations for charging profiles and match it to the regulator.
AGMs accept a much higher amperage and many alternators are not up to the task and will burn out. Similarly, ensure that cabling is able to handle the increased amperage and that your OCP (over current protection) is designed to up to date standards. Then there is a review to ensure the belt(s) powering the alternator are up the task of the increased load on the alternator and that the loads do not ask too much of the PTO (power take off) of your propulsion engine. I consider the propulsion engine “mission critical” so I shy away from asking too much of its ancillary duties for worry of compromising its main function.
For many, the upgrade you are considering leads to a complete upgrade of the whole charging system of the boat.
Much of the above can be mitigated by a good regulator. Balmar (among other manufacturers) makes one where amperage can be limited and where the regulator can monitor battery temperature as well as alternator temperature; both are wise safety options. But to get the most out of your AGMs, it is better to have the whole charging system upgraded.
Having good consult is important and you say there is none where you are. This is a complicated area where significant damage to components and/or fire might occur inadvertently. By far the best series of articles on the care and feeding of AGMs is on the Attainable Adventure Cruising web site (https://www.morganscloud.com/).
Visiting this site will entail a ~~$20 fee, but any sailor will benefit far beyond the $20 very quickly.
Another alternative is to find a professional marine electrical engineer (in the US I would insist on ABYC certification) and ask him/her to consult from a distance and possible pay for a visit at the end to “sign off” on the changes or take the boat to him/her early the next season.
I am sure I missed important elements: please feel free to come back with questions/comments/thoughts.
My best, Dick Stevenson, s/v Alchemy