Thanks for the comments.
Yes I can rename the file extensions. What concerns me though is the effect on SEO having changed them.
The sites been going for 10years now and I have an awful lot of inbound links all over the interwebs that I have no control over. Those links will result in 404s, as will any pages book marked by anyone. This will also be very damaging for any search engine results, who will see it as a brand new page and ill lose any ranking.
You can do a 301 re directive which greatly lessens the damage, as far as bookmarks and old inbound links are concerned, although it still apparently leaves some residual damage with the likes of google - who apparently can penalise. This is because of misuse of 301s in the past by unscrupulous SEO optimising ype agencies/persons, using it to exploit what it's really there for.
I'm also not looking forward to compiling 301s for a few hundred pages.
I believe there is another option and that's getting my server to do all the work. Apparently apache there's an apache command that can be issued that forces my server to parse and treat any HTML pages as if they were in the language you specify. This seems like it may be a way forward, but one of the downsides it additional load on the server, as from from what I understand, pages are parsed server side rather than the more normal client side (ie the browser).
As the interwebs progress, more sites are now becoming dynamic for various reasons which is why larger sites are now moving to extensionless pages. It becomes a real pita when technologies change. When the site started I used cgi, because that was the way then to write any server sides, I gradually moved to php and used 301 redirects but not that many pages were involved.
Extensions are a pita, what with .htm .html. .php .jsp .asp .cgi (and more). The actual content may remain the same, the page name may still be the same,but as far as google and browsers are concerned, change the extension and its a brand new page.
I'm not sure yet if I fully understand extensionless pages as doing this it itself could cause work (when I suss out how to do it), but I can understand why the larger sites are now doing this when different technologies such as jsp and asp make way for new languages. Get the hard work over and done with once and for all. Certain mobile devices ( ie apple) not allowing certain scripts and extensions which many sites use for menus is another headache
Php though will (hopefully) be around a long time because its used by a lot of forum software and cms systems.
Anyhow the crux is making a decision on which way to progress now, and what effect it will have on the site for inbound links. It may seem simples for a team of web developers, but I'm just a jack of all trades, master of none, doing what I can