Replacing subfloor boards, which wood?

ThisIsWeird

Is there a flexible levelling resin that can be used over timber?
What do pros do in such cases? Even if they line with screwed ply sheets, it'll still surely dip?

danrv

grumbler said:

In modern houses they use big tong&grove chipboard instead of timber boards. 

That’s what I’ve used for my bathroom project.
https://www.wickes.co.uk/Wickes-P5-Tongue-and-Groove-Chipboard-Flooring---18-x-600-x-2400mm/p/164516?fix&gclid=EAIaIQobChMIjNSAq7uBgQMVx9_tCh3pmwbkEAQYASABEgLjG_D_BwE&gclsrc=aw.ds

Cheap, solid and screwed down for the loose lay LVT top floor. Plywood may have been better but pricey for the marine type.

OP, if most of the floor is level and doesn’t flex or creak then just replace the problem boards, like for like. Plane off or add to the joists to get the new boards level. Maybe an electric planer or sander to help get the raised floorboards level or some compound to bring the level up where needed. Otherwise, if too much work is involved, see the if the current boards will come up fairly easily. Obviously take care with pipes and cables underneath. 
Large chipboard or ply sheets would much more suitable for the chosen top floor.

pieroabcd

Now it's all level, except a couple of spots where I have to plane the subfloor boards by 3mm and another one where there are 2 joists side by side: one of them is 3mm higher than the other, by something like 20cm of lenght.
This has created a cusp in the overstanding large plywood board.
Is it safe to level that joist by 3mm? I would prefer even more to plane the plywood, but is there a risk that it will break?
I have an oscillating tool that seems to do a decent planing job.

In all other places the creaks were resolved by addinh screws to the joists. The existing pipework and electric wires are in safe positions and repaired from mails and screws, with adhesive tapes on the surface to indicate where they are.