You should be safe... it only removes and install qmail as on a new installation!
Personnaly, after reading your other post (spamdyke), I would do so to bring back qmail.
Plesk 9 and qmail-scanner
Re: Plesk 9 and qmail-scanner
thanks Albans. I found the bit in /etc/xinetd.d/smtp_psa that was preventing it, and now it is all working - thank you.