I wouldn't worry, Plasmas are said to only have a lifespan of 5 years!
HD is still in it's infancy, SKY being the only provider at present with a very limited range of channels and a £10 per month surcharge. And HD-DVD is still fighting it out with Blu-Ray as to which will take over from regular DVDs.
So by the time HD broadcasting is widespread, on all platforms and affordable, and a replacement HD standard for DVDs is agreed upon, your Plasma TV will be on it's last legs anyway (and probably suffering screenburn and other problems).
Then you can buy an HD LCD - which will have come down in price too.