The only players that were made that could play both Blu-ray and HD-DVD were three players made by LG (BH100, BH200) and Samsung (BD-UP5000) that were specifically designed to play both formats in the same machine. Since HD-DVD is no longer available, the combo player concept was also discontinued in early 2008.
Other alternatives that have been proposed to solve this problem include a dual Blu-ray/HD-DVD disc. This would be a disc that would have the Blu-ray version of the film or program on one side and the HD-DVD content on the other side. A similar solution is already being provided on some HD-DVD titles, in which the HD-DVD content is on one side of the disc and standard DVD content is offered on the other side.
However, so far, both Blu-ray and HD-DVD player manufacturers have included the ability for their units to playback standard DVDs. This means your current DVD library is playable on either a Blu-ray Disc or HD-DVD player.