I might be wrong, but I believe that since your board is older, it gets "grandfathered" in to ISAF approval
if it was approved and registered at the time it was manufactured.
Actually, where are you located.... that would be a good question to ask the yachting authority (the group with a member on the ISAF council) in your country.
It may be different in different parts of the world.
I've never been to a race (all over the USA at all levels, and in Thailand) where they checked the ISAF registration numbers on boards.
Where you are it may be different.
If they ask you where your ISAF registration/approval is..... show them your board.... it's printed on the board, right?