You could do it, but what happens if the master's database was changed in the meantime?
If I were you, I would replicate the data on slave and then, on slave, do the whole procedure: change indexes, stop slapd, reindex, start slapd, just to have a time estimation of the whole thing.
So you can do the job on master at a time that will not affect your clients.
Or if you feel risky enough, you can use the slave to replicate the newly indexed data to master.
No need to mention that you should backup before anything else.