Worth upgrading tonearm cable?

Any, the 600 is better but more expensive. There is a new 250 with VTA adjustment for $350. The lack of VTA adjustment is the only problem with the Rega and it can be added as it is here. I cannot say about the arm board; it may fit in the same cutout. Check the length of each arm. If the SOTA uses a wood board a woodworking shop can make a new one and cut it for new arm if you need a new one.

The Rega is a slightly longer tonearm than the Linn, roughly 237-240 mm for the Rega vs. 229 mm for the Linn. If you switch tonearms you'll need a new arm board.

And is the Rega substantially better than the Linn?

I should have specified my reasons for recommending it. The Rega is cast as one piece out of magnesium. SME also does this but no one remotely close to Rega's selling price does. This produces a much more rigid, resonance free arm. Rega can only afford this because of economies of scale as they not only produce arms for themselves but for many other turntable companies. It is safe to say that they are essentially the default choice for those who do not wish to spend over $2000 on an arm. I have used them myself at a time when I was an SME dealer. The SME was better but the Rega was not embarrassed by the comparison.

I was talking about the Rega arms at a local dealer the other day. Seems almost nobody sells arms separate from the tables anymore, but “he could get me one.” I'd also need a new Rega-cut arm mounting board for the SOTA table and, apparently, some shims for the arm, which I hear has no vertical adjustment. Another possibility would be to buy a better Linn arm, used... Then I wouldn't have to mess with a new arm board for the 'table. Thoughts?