If it is a field gun in shooter grade condition I would not hesitate to open the choke. I would do whatever it took to make the shotgun usable, otherwise it is just a paperweight. There were quite a few Belgium made A5 shotguns so it is not all that rare.
If it is a higher grade and it is in a condition that a collector may be interested in (safe queen with highly figured wood) I would not touch it.
As to the original question - yes it is possible to open it up yourself but you would stand an excellent chance of ruining the barrel. Take it to a gunsmith that has actual experience (not just something they have read about) in opening chokes on older shotguns. Some, maybe most, gunsmiths have no real "hands on" experience with this. Someone at the local skeet range probably knows a good shotgun gunsmith.