As per title, pretty silly question really, but are 29” and 650B+ interchangeable?

My dads got a 650b+ eBike and wants another set of wheels with skinnier tyres
Can he put a pair of 29” wheels on instead, and might this actually be better as his speedo would be configured for a larger diameter?

He’s also thinking about upgrading his forks, would he be looking for a set of 29” and do they need to be ebike specific? (Some forks say they’re meant for ebikes)

Would these be ok, they say eMTB in the blurb but not the title, and I know crc can be very generic descriptions for stuff

https://www.chainreactioncycles.com/mobile/rockshox-yari-rc-boost-fork-2020/rp-prod208349?utm_source=affiliate-window&utm_medium=affiliates&utm_campaign=Skimlinks&utm_term=Skimlinks&utm_content=Sub+Networks&sv_campaign_id=78888&sv_tax1=affiliate&sv_tax2=375403%7C375413%7C375415&sv_tax3=Skimlinks&sv_tax4=singletrackworld.com&sv_affiliate_id=78888&awc=2698_1642848429_313ea2200ead63068e0dfa4b91cdadc0&sv_campaign_id=78888&sv_tax1=affiliate&sv_tax2=375403%7C375413%7C375415&sv_tax3=Skimlinks&sv_tax4=0&sv_affiliate_id=78888

Also his current forks are 51mm offset and the ones above are 42mm
I don’t really understand what offset means, so don’t know if this would be a problem
is fork offset a newish thing? I’ve been away from biking about 5 years and don’t really remember ever considering it when buying forks in the past

Will depend on the frame clearance on the rear.
We have Turbo Levos, I run mine with 2.8 x 27.5 and daughter runs hers with 2.6 front and 2.3 rear 29.
No problems at all.
Non of them have ebike specific forks, wheels or drivetrain.

This is how they look

He will probably find his current forks are a 29″ model. Forks will take 29″ wheels, frame probably will just need to check the clearance.

I had a Scott 27.5 plus bike that came with 2.8 tyres. I used to run that with 29″2.35 tyres with plenty of clearance and no issues whatso ever. The only thing it changed was about 5mm more clearance under the cranks which was a huge improvement over standard. Just go for it I say.

I have a 27.5″ hardtail with Fox 32 forks. I could just squeeze 29 x 2.00 tyres in, but the clearance was so tight that I went back to 27.5 with 2.35 tyres. The only way to tell for your particular frame and forks is to borrow some wheels and see how much clearance there is.

What bike is it

I have a 27.5″ hardtail with Fox 32 forks.

27.5+ does not normally fit 27.5 forks.

27.5+ does not normally fit 27.5 forks.

Oops, missed the plus bit.

What bike is it

It’s a voodoo Zobop ebike from Halfords, so nothing special
He hasn’t ridden a bike since he was a teenager though and is now out multiple times a week, he absolutely loves it

29er wheels are a little taller than 27.5+, maybe 10mm or so (with variations depending on tyres)

As a general rule they are interchangeable, but you might find on some bikes the wider plus tyre or taller 29 causes some issues. Check how much space there is in front of your existing tyre at the BB and seatstay bridge

Front should be OK with 29er clearance.
It may be worthwhile going back to Halfords to see if they have a rear 29 wheel and tyre they could try in. I’ve always found our local Halfords to be quite accommodating.
What width tyres is he wanting to fit and why