New envelopes are formatted with the Envelope Address and Envelope Return paragraph styles in the document template. If you have hundreds of templates, that's a lot of styles to modify.
It sounds like you are creating new envelopes from old documents. In that case it is simpler to use an envelope template which will remain the same every time it is used. To this end you will find
http://www.gmayor.com/Envelope_Label_Add_In.htm useful.