Normally if you put 'copyright' and the date on a drawing you will be able to claim ownership if someone else copies it and uses it at a later date.

and/or make a print and mail it to yourself (registered) and don't open the mail when you receive it ...

but ... not many changes are required by the copier to void the copyright ... that varies from country to country.

If you are really concerned then my best advice is to contact a copyright lawyer for advice.

cheers,
eric