Planned Order Conversion to Process Orders

First ensure that setting in the material master for BOM explosion is "Selection thru prod version".
Then Pls check the validity periods of the new production version.
e.g Prod Ver Val frm val to No lot size variation 0001 01.01.2007 31.12.9999 0002 01.01.2007 31.01.2007 or 0001 01.01.2007 31.12.9999 0002 01.01.2007 31.12.2007
Then in both the cases system will always pick prod ver 0001 default (Longer validity period). If not the case then Check for lot size variation (if lot size dependent.)
Further if you are not able to select the req prod Ver then you can lock the earlier prod version(if it is not in use).
To avoid this prob in future.... always use "Valid to date" either in limits (31.12.2007) or without limit (31.12.9999) for all prod version of a material.