When you go into Change entries in GL, are you selecting the new or current GL Transaction file?
If it won't post, it should be in the new.glt.

In File - Company Settings - GL Settings under the Processing Options there is a box which normally isn't checked which allows you to edit transactions created outside of GL. I strongly suggest that you should consider correcting the error in the application of origin (i.e. AP, or BL, or AR, or etc.) and that way the corrected date will be fixed everywhere. If you choose to edit the transaction in GL, only GL will be corrected but no other place will be corrected. To edit in GL only, you'll need to check the Processing Options box before you can see the journal entry. Remember to uncheck the box when you are through.