Thanks for posting in the Community, dorretti.

Can I ask what specific GST code and transaction are you trying to change? There was a reported product investigation last month about Unable to reclassify GST on deposit transaction. And the engineers mentioned that bulk edit of GST on purchases to sales type is not allowed in QuickBooks Online. 

If the transaction is of sales type and the user is trying to change it to a purchase type GST then it's not allowed.
But it's allowed from the actual QBO transaction page.

Deposit-type transactions are not considered purchase- or sale-type transactions, but once GST is assigned to a deposit-type transaction, it becomes a purchase- or sale-type transaction based on the GST type, and hence, changing tax type in bulkEdit is restricted.

If you still need more help with reclassifying your transaction. I recommend also checking in with our Customer Care Team. That way, they can check your transaction and ask for your details in a secure form.

Drop by again in the Community if you have other questions.