To identify how many reports will be impacted after the Unicode conversion you have a SAP standard Tcode "UCCHECK". You can use this transaction only in unicode compliant system i.e. 620 (R/3 4.7) & onwards.
If your customer is with release 4.6C or lower then you have to do the Sandbox system upgrade first & then find the impact.
Hope this will help you, do revert if you need any more details.