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.
“I haven’t raced with MJ before but we shared Century’s garage and motorhome last year so know each other pretty well.
“It’s not like we’re starting from square one, and obviously I know the car and team.
“We’re really lucky to be starting from such a strong position given how late this has come together.”
Harper's addition to the grid ensures that there are six previous title-winners in the 19-car GT3 field.
This includes four-time British GT champion Jonny Adam, back racing an Aston Martin for the new Blackthorn team with Giacomo Petrobelli after spending last season in a Mercedes, 2022 champion Ian Loggie in a 2 Seas Mercedes alongside Phil Keen and two-time champion Andrew Howard in a Beeachdean Aston Martin shared with Jessica Hawkins.
The 2020 British GT champions Rob Collard and Sandy Mitchell are back too and line up on opposite sides of the Barwell Lamborghini garage, sharing with Ricky Collard and Alex Martin respectively.
It was announced earlier today that Century will also contest the GTWCE Endurance Cup, with Leung joined by Formula E world champion Jake Dennis and Toby Sowery.
