According to some leaked intel (and some partial corroboration from the company themselves), HTC's upgrade plans for their two flagships, the HTC One M7 and M8 are now known. First, HTC themselves shared they plan to upgrade both devices to Android L within 90 days of the first launch of the OS. In the interim, some insider info filtered out suggesting that they will be moving to the Android 4.4.4 update sometime soon. This will likely happen in the July-August period.
The leaked chart which reveals this info also shared details regarding a few other HTC products. Sadly, the news is grim as it looks like many of them will not be updated further.
Source: llabtoofer