existing light (four wires) into new Led light

[Jock from the north]

If the hole in the ceiling is large enough to push the box through (and remove it), I would class it as accessible.

I guess anyone looking for a fault would guess there is a jb above light

 

[brianmoooore]

There's plenty of flat area inside that fitting to write "Remove to access JB above" in permanent marker.

 

[westward10]

Plenty of room to tidily fit some Wagos.

 

[brianmoooore]

Plenty of room to tidily fit some Wagos.

There is, but there isn't room to get the four cables through the existing cable entry, so the fitting will need modifying, and that opens up a whole another can of worms.
Three lights suggests a fairly long corridor, with similarly long strappers for the two way switching, so plenty of capacitance between them. I won't be surprised if the OP comes back with a question along the lines of "how can I stop my corridor lights glowing when off/doing disco light impressions?"