Oh dear, this has the potential to end badly, before embarking on anything like that you MUST do insulation resistance tests, and that's before you even agree to doing a board change. There are so many potential problems that can come back to bite you, as this job appears to have done.
Potentially if the original board had no RCD protection then you could be looking at any, or a combination of the following
- low IR value between live and earth
- low IR value between neutral and earth
- a dead short between neutral and earth
- damp inside a fitting
something connected across 2 circuits, I've seen it where a light has taken it's live from the lighting circuit and it's neutral from the ring circuit. there are any number of problems that could cause the situation you have, if you are lucky it will be something you can locate and rectify fairly quickly and easily, if not then you could be heading for a world of hurt.
Don't take this the wrong way, but I'm thinking you don't have much experience, and believe it or not, there are people who'll take advantage of you, "well, it was working before you touched it" is the usual one, then they expect you to repair a fault that was actually there before you started, however, you didn't do the proper tests, so you can't prove it was there.
If I come across a job like this, the first thing I do before even agreeing to do the job is a series of tests, if any come back wrong then I tell the potential client what the problem is and try and give them an idea as to what it may cost, although this isn't always easy. If they are not prepared to pay for the corrective work done, before doing a board change, then I'll walk away, it's just not worth the hassle in the long run.