Safe to change 3 year old to high back booster?

Question

My 3 year old is currently using the joei 360 spin car seat with 5 strap harness.
He's 35lbs and he's quite tall (75th percentile for his height).
I was thinking of buying a stage 2/3 high back booster which can use from age 3 that just use the seat belt in the car.
Is the high back booster with seat belt as safe as the 5 strap harness? What would you recommend and when did you change your 3 year olds seat?

Answer

A hold should not be out of a 5 per harness til 5 years at the earliest. And only then if they can sit responsibly. 
Before 5, their Skeleton is just not ready for the forces they would receive from just a belt in a crash. 
Leave them in a harness.

Answer

This is what I have. After the 3rd setting, it’s usable with just the seatbelt and then it changes to no back booster seat.

Answer

Absolutely not. Heck if it was me, I’d still have him rear-facing.

75th percentile is not crazy tall, I’m sure many seats could still accommodate him rear-facing, and he certainly has plenty of time left before he reaches any max limits for forward facing with a 5 point harness. 
There is absolutely no reason to switch him to a booster.

Answer

No no. 3 year old should still be using a 5 point harness. I have my son in a ‘high back’ booster but it has the harness. And then he’ll use the seatbelt when he’s older.

Answer

No, the child should be at least 4, 40 inches tall, and 40 lbs.

Answer

So I’m realizing you must not be in the US, and that the seat you currently have has. Roughly 40lb limit (18kgs) for both rear and forward facing. It seems this is common for some European seat, which is odd compared to the US where many have a 40lb limit for rear facing and then a 65lb limit for forward facing with a 5point harness. 
Anyways, I understand more now you looking at boosters as your child is about to outgrow their seat either direction. But, given his age and size he will be much safer in a 5 point harness than he would in a booster with seatbelt.
I suggest you look at something like the Joie Bold which has a higher forward facing harnessed weight limit (25kg/55lbs) and the ability to later convert to a booster when he is older and meets all the requirements* for a booster seat with seatbelt. 
*by requirements I mean the recommended safety requirements set by child passenger safety technicians (shared in my other comment) and not the legal requirements which seem to always fall short of what is actually the safest choice.

Answer

This is why it's confused me as this is printed on the boxes and I thought seemed a bit soon. His current joie 360 spin seat goes to 40lbs so I will keep him in that longer.