Vectors layers wont merge - integer instead of integer 64 error. Refactor fails to change

I encountered the same problem. I was successful with the following workaround:

• Open the attribute table of integer64 layer
• Create a new column and enter data -- I just entered an arbitrary ID -- selecting integer and making sure the length is less than 10 (as below):

• Select the features from the layer you want to merge, right click the layer and go to Export > Save Selected Features As...
• From here, you can check or uncheck the attributes you want to save. When I exported only "id3" and "integer_te" the vector layer had the same shape data but the layer was now classed as integer, rather than integer64.

The merge then ran successfully with this new layer.