I have got a Yanmar 3GM30 with the standard internally regulated alternator. I know smart external ones are much better. Question: Can I "upgrade" to an external one or do I have to dump the existing one and get a new alternator as well?
No you don't have to dump your existing alternator. I had a Yanmar 2GM fitted with a TWC regulator. The internal regulator is simply bypassed and the output from the alternator goes directly into the TWC which controls the amount of charging current sent to the batteries. Devices such as Ardverc do a similar job.
It's fairly easy to add a smart regulator to your existing alternator, and it will help to ensure that your batteries are recharged as efficiently as possible.
Adverc BM (www.adverc.co.uk) originally marketed the TWC regulator in the UK and now produce their own regulator. I've fitted their products on 3 boats, with good results. One advantage of the current Adverc regulator, compared with some competitors, is that it works in parallel with your existing internal alternator regulator - this means that, in the unlikely event of the Adverc unit failing, your internal regulator will continue to provide charging. The Adverc website has lots of information, and you'll find them very helpful people on the phone too.
Drifgate 2000 (DG2K.co.uk) do a similar regulator to both the Adverc and TWC. I have had a DG2K "Skipper" regulator on my boat for four years - and it is a fit and forget item. The batteries are then as fully charged as possible.
Look through PBO and contact all the advertisers of alternator regulators to compare product and prices - looking at it now, DG2K in page26, Adverc page 31 are advertising.
If you want my installation drawing - alternator booster, splitter, three batteries, belt and braces isolators, voltmeters etc, etc, contact me via PBO emails.