Changing domain name a risk?

I've just found this forum and I hope you can advise me. I'm reasonably familiar with on-page SEO but I'm unsure of a lot of the off-page SEO stuff.

I'm a UK website designer.

I have a new client whose site is currently one of those hosting company build-it-yourself sites which is appearing precisely no-where for the very competitive keywords she wants to go after. Of course I'd rebuild her site so that it would be more attractive to search engines and customers alike.

I've found a slightly less competitive key word/phrase - still very competitive but less than the main one. The thing is she owns a domain name that basically IS that phrase. That domain name has been just sat there since 2009 - not even forwarding to her current domain name.

Her current site has a PR of 2, has been around for quite a few years and quite a few backlinks mainly from what I can see from authoritative forums in her field.

So if I start again and build her a decent site on the keyword domain name and then 301/park the old domain name onto the new site will I keep the benefits of her current PR2 site. Plus what about all those backlinks to the current domain name?

It occurs to me that since a lot of them are on a forum she could go into her profile and just change the URL in her signature which would then surely create a load of backlinks to the new site.

Just a little confused about whether to stick with the old, recognised domain name or go with the domain name for the very competitive area.

thanks for reading!