I'm no paint expert, but I know that some paints don't always adhere well to others of a differnet type, I also seem to recall my NB surveyer stating the same and advising that if you can't identify what type of paint your boat has then play safe and have it gritblasted back to bare metal and start again. I know that will make it more expensive in the short term but perhaps that is better than spending a lot of time, effort and cost, only to find it all peeled off within a month or two because of an incompatabilty with the old coating.
Regarding bottom plate painting, we are going to have ours done [only just bought her] as she was until recently on a permanent mooring and hasn't been painted for four years. The bottom has large oyster shaped scabs which the surveyer reckons where caused by lack of movement but which are quite deep and will benefit from painting. So, while having the sides grit blasted we shall try to get her high enough off the ground and get the bottom grit blasted as well. Shouldn't be a problem, just a matter of putting enough blocks safely under her to give sufficient space to work. I'll let you know in 12months if it was worth the effort, bet you can't wait, can you???????
Regarding anodes, there are many causes that can affect their performance and they can include what type of mooring you are on [metal or wood] whether or not you use landline power and what type of earthing yours and other surrounding boats have, if they have not had their earthing properly installed IT WILL AFFECT YOUR HULL SERIOUSLY.
http://www.canaljunction.com/british/waterways.htm
has a lot of useful stuff about this problem as well as other very good info.
Hope this is of help to someone.
Great site, found some really helpful info on here, and seems to prove the rumour that NB'ers are good people.
John