Dear Saly,
as correctly said by Caetano in standard SAP the only tool to change work center in production orders is in leveling activity through CM21 (graphics or tabelar - CM21 , CM22).
In this strument (CM21) according to customizing setting in strategy profile (belonging to overall profile)
you can change resource through drag / drop method or (with special setting in strategy profile) insering manually a new resourse.
As alternative tool is possible to create a custom report and change work center using a BI custom using a filter tool present in the production order after go to operation details. Unfortunaltely in the standard SAP there isn't a bapi/fm (released by SAP) to change work center in production order.
Let me know if i've been useful for you.
Daniele