I have a USDM Zillion SV TW G in the 6.3 gear ratio, and I have a JDM Zillion TW HD 1000 in the 8.1 gear ratio. This may be a silly question, but is it possible to swap the gears between the two?

I gave it a shot and it seems to be a simple swap, with the disclaimer that I don't really know what I'm doing. I swapped the pinion gear + drive gear between the two, and I'm going to try it out on the water tomorrow or this weekend. Looking to use this TW HD with the low-speed gearing as a chatterbait reel. I have been using a Metanium 7.1 on a MH/F Expride, and have always said that I prefer a 7 speed for a chatterbait, but the MH/MF Expride glass comes in tomorrow and I'm thinking the slower action may be complemented more by the slower ratio reel.

How did this turn out?  Any issues?   I am thinking of putting 6.3 gears in my zillion hd because the 8.1 is too fast for what I want to use it for.  

No issues. I'm still running this setup. I caught some decent-sized bass on the Zillion HD with the 6.3 gears last summer. I've barely fished at all in 2024 but that's a "me" problem.

Nice nice.    I have a usdm Zillion SV 6.3 so I’ll have to do the swap.   Glad to hear it’s still working good.    The zillion HD is awesome but really too fast for what I want to use it for