Hi, myabdlsupplies.
At this time, the option to change the GST tax codes in batch is currently unavailable in QuickBooks Online so it has to be done manually.
QuickBooks aims to find new ways to make sure that your product meets your needs, I'll be taking note of this product preferences. I'd also encourage you to check our QuickBooks New Features to be updated on the latest product enhancements.
Additionally, you can visit our Community Help Articles for some "How do I" steps in QuickBooks Online.
Please let me know if you have any other questions. I'm always here to help and provide you additional assistance. Keep safe!