I have no idea but if you have email support you might want to start the dialogue with Google about the issue before you follow through with it. Maybe there's a way they can associate accounts or something on their end so they can see the history?

My earnings did not suffer in the slightest when I went from a personal account to a company account in 2010.

I'm moving some sites from one of my companies to another company I control and just opened a new Adsense account about an hour ago to get the revenue going to the right payee (of course waiting for approval). Will try to remember to come back here and update you on what actually happens once the new account is in place and ad-codes are updated. This is one of those situations where it doesn't matter whether there as any such ramifications or not. Sometimes you just gotta do what you gotta do.

I have been in touch with G already because I'm desperately trying to avoid having to open a new account - it's certainly not through choice I'm doing it! They say it's the only option...

The "only option" answer is publicly available in the documentation for Adsense. I believe it's tied to the fact that they don't want people getting in the business of selling Adsense Accounts or operating Adsense on behalf of third parties. Only the party that created the account is entitled to collect the revenue from it according to Google's laws. I understand this but it's also a pain for someone with a personal account who simply now wants to put their hard work under their newly formed business name. Been there, done that. The moral of this story is to think through your business model before creating an Adsense account. When your business model changes, well, you're stuck with Google's business model and that's just the way it is.

Changing accounts is a pain. Good luck. In my case my business changed from my personal name into my business name. Rather than create a new business AdSense account, I just updated the name and bank account info. Technically it's still a "personal" account as far as Google is concerned but it doesn't matter to me. Shhhh...

yeah, in my case, it's nothing to do with names, but of countries....

Well, I'll agree that the process of setting up a new Adsense account is now officially a "pain" compared to the past. I've been waiting for three days just for Google to recognize that I've place the code on my site.

UPDATE: It took exactly 7 days to get a new Adsense account approved (complete with bank account verified). The first day with new ads (swapping out adds from another account on an established site) proved fruitful. I will say that in the process of swapping ads, I went ahead and took advantage of Smart Sizing and some ad relocation which has had a positive effect on production on some historically low-performing pages. Overall, this necessary experiment is proving that switching accounts shouldn't be too devastating on an established site that is in good standing. The one caveat here would be that the experiment is only a couple of days old so who knows if there will be a dip at some point but, so far, all looks normal--if not a bit better than expected.