Hi John,
Thank you for posting query to us.
The DataSource for a specific cell can be changed dynamically by handling the CuurentCellShowingDropDown event.
Please refer the sample which implements the above said feature:
http://websamples.syncfusion.com/samples/Grid.Windows/F68654/main.htmKindly let us know if you need any further assistance.
Thank you for using Syncfusion Products.
Best Regards,
Jeba.