In beanhead's scenario of 1.2M dropping to 1M in the NISA. If it then rises back to 1.2M in your taxable account, you pay tax on the 200k, even though you didn't make any money. Whereas if you'd sold it before it expires in anticipation of this, you'd miss out on the 200k gain at all. In both cases you're worse off due to the timing, than you would be if the NISA hadn't expired and you could just ride it out.
a. you have unallocated space new NISA so it doesn't grow in taxable account.
or
b. you don't have space so you benefit form buying more units at a lowered value.
a) That's true. But there is still out of market risk, so the price could go up between your sell + buy order. It's probably not a major risk, but it's still there.
b) This doesn't work - now you have 1M of taxable assets, and have to pay gains on that, even though you invested 1.2M.
Something is getting lost in translation here. Best to leave it now…
