I bought a house built in the US in the mid 70s, and the vents for the forced air system are mostly at the baseboard level.
The original vent covers were something like this, like a wall register but with almost an inch of projection from the wall, but the register bottom was on the subfloor, below the level of the carpet.
We've replaced the carpet floors with LVT, which involved adding 1/2 plywood on the existing subfloor, and now the duct openings are level with or actually a hair below the flooring.
This isn't a big deal for the returns, but the supply vents need louvers, which project a bit back into the vent opening, and the margin that these generally have on the bottom now means the top of the louvers are above the top of the vent opening, and so they won't slide in.
I think I can cut the bottoms off of the registers with an angle grinder or jigsaw, but am wondering if there's a less destructive or more standard solution for this issue.