Isolating income/gains, "ring fencing"

The mutual fund to ETF conversion is just a name change as the assets are exactly the same. You'd have to convert to a different ETF from the mutual fund to crystalize the capital gains. Personally I'd do that to make things simple and to give me freedom to access all my funds in a tax efficient way......of course you also need to consider the tax bill for such a one time sale.

I think that's right, but it seems like there is a catch/trade-off. Vanguard says that you can convert to an ETF tax-free. Presumably that means that capital gains don't "start over" from zero. So even if I sequester the new income into another fund, when I sell the ETF, I'll have to pay capital gains starting from before my move, and it'll be really complex to figure out.

Alternatively, I could incur the tax now and "start over" from zero. Presumably I could sell of the mutual fund and buy the ETF (keeping in mind the rules for "wash sales"). But the downside is that I'd have to pay any capital gains taxes now, which is disadvantageous (e.g. due to compounding and the time value of money). I'm pretty sure I want to avoid that, especially since I don't know how much of my investments I'm going to want to sell in the future while I'm in the UK. (We're not sure if we're going to be there for a long time or not.)

Does that sound right?

Yes, you have it right.

When I did this prior to our move this year the index mutual funds transfered to the equivalent ETFs with no tax owed and the unrealized capital gains intact, but I did have a managed fund with Vanguard, a mixture of stocks and bonds, and I sold that over a 3 year period paying cap gains each year and using the proceeds to buy shares in an ETF, (VYM) which invests in dividend yielding stocks and is an HMC Reporting fund so the dividends will be recognized as stock dividends.