Replace section of 16mm T&G chipboard floor

[Cauliflower]

Due to a leak (pipes running under my hallway - one developed a pinhole leak ), I (actually someone else) took up the hallway carpet and cut out a section of the hallway flooring. It is a 1980s first floor flat, floor is concrete bottom, then polystyrene insulation (25mm), and then what appears to be 16mm (or 15mm?) chipboard under the carpet underlay. Leak is fixed but now I need to replace the water-damaged chipboard. I actually want to replace it with something vaguely water resistant like a plywood, but anyway, 16mm doesn't seem to be a thing any more. Most of the available flooring seems to be 18mm now.

So my question is, I know it's only a 2mm difference, but the new section of boarding has got to butt up against the existing chipboard halfway down the hallway where old will meet new (and has to fit below the skirting boards on either side (or at least butt up against the existing chipboard that still exists under those skirting boards). Is the depth difference going to be a noticeable thing, does anyone have any sneaky tips to make the two different board depths match in a nice seamless way?

The new section is roughly 1800mm x 900mm

Thanks,

 

[blup]

Get 15mm and add underlay to build up size as necessary. Try osb or ply if you can’t source it in chipboard.

Blup