Harper and Darren Leung took victories at Silverstone and Brands Hatch on their way to the title in 2023 and the pair will reprise their partnership in GT World Challenge Europe Sprint Cup with Century.
But Northern Irishman Harper will now step into the British GT seat originally earmarked for Chris Salkeld and partner Michael Johnston in an M4 GT3.
Salkeld and Johnston had planned to step together to the GT3 class, after claiming the GT4 pro-am title with Century last season.
Carl Cavers and Lewis Plato will drive Century's second GT3 entry in Silver-Am.
It remains unclear what prompted the team to make a change so late in the off-season, as the championship is due to commence this weekend with a double-header at Oulton Park.
Autosport has approached Century for comment.
“I’ve got to be honest: British GT wasn’t on my radar for this year, even yesterday afternoon!” said Harper.
“I’ve got a busy year with BMW ahead, but by complete chance didn’t have a clashing commitment elsewhere. Perhaps it was meant to be.
Source: Autosport