HTC has confirmed on Twitter that the HTC One M8 will indeed receive updates to bump the smartphone up to Android 5.1 and Sense 7 in August later this year.
Sense 7 was launched by the company alongside the HTC One M9, but this future update will bring the predecessor up to speed with the latest flagship smartphone, not only regarding Sense UI but also Android too.
@JKMcCloskey Hey James, the HTC One M8 will have a Software Update for Android Version 5.1 and Sense 7 in August 2015 :)— HTC UK (@HTC_UK) April 29, 2015
The date was provided by HTC's UK Twitter account, and thus we caution that it may apply to HTC One M8 owners only in the UK. We'll hopefully have more details to share for other markets.
Source: Twitter (HTC UK)