oh fair enough if you just meant for now...
basically, it should be any pricing-related table that has a 'decimal' type that currently uses 10,2 - if you write a SQL query to identify these tables, it will give you the following list (from v7.6b1 but I don't think there are any new tables in v7.6).. showing database table name and the decimal field(s) in that table to be edited.
you could probably exclude changing tbltax => taxrate from the list - unless you need to add more decimal places to a tax rate e.g 17.555%... other than that reason, I don't think it's relevant to change tbltax => taxrate.
no Kian said that, not me!
no guarantees, but you should not.
that's a legitimate concern, but if support have been suggesting this for years (unless there are elements in there going rogue lol), then I would think this to be low-risk in the long-term, but again no guarantees...
although absolutely the best solution would be for WHMCS to do this themselves... I can't for the life of me see why they haven't done it already as they must be getting support tickets from users in countries where this would be applicable to their currencies.