Do the slapadd tool keep the operational attributes values ?
It does not. See the slapadd man page.
If we try and copy the database, shall we see instantly if there is a problem, or can it occur at any time ?
slapadd has very little error checking versus adding through slapd. The restrictions are detailed in the slapadd man page. If you feed slapadd invalid data, but not so grossly invalid that slapadd errors out, it is quite possible that lingering effects (missing parents, schema violations, etc.) will "occur at any time" in the future. You basically want your data to be clean enough for ldapadd, whether you use slapadd or not.
By the way, if you're going to be migrating anyway, you really don't want to migrate from 2.2 to 2.2. Try the latest version of 2.3 available at www.openldap.org.