Your 401k dollars are pre-tax dollars. Rolling them into a rollover IRA is straightforward, and not a taxable event. Let them grow in the rollover IRA account.
Converting tIRA dollars to Roth IRA dollars usually *IS* a taxable event, and increases your MAGI. Many people save up in taxable accounts to cover expenses during the first 5 years of FIRE, and use those years of low income (and low tax bracket) to do the conversion.
Here's my understanding of it. I did this 2 years ago during a full-earning year, (I'm pre-FIRE) converted $39k into my Roth and ended up paying more taxes than it was worth. If I had it to do over again, I would have waited until after FIRE to start the ladder. Here are two examples - hopefully this will help. Single retiree will convert $30K/year for expenses. Tax brackets are different for married filing jointly (YMMV). [MDM, and others... please correct any mistakes]
Example 1:
Start today. Pre-conversion MAGI $120k. Post conversion MAGI $150k ($120K + $30K conversion). 28% tax bracket. Tax=$8,400
Example 2:
Start AFTER FIRE. Pre-conversion MAGI $0k by living off long-term capital gains from a taxable account - will pay cap gains of 0% if selling funds purchased >1 year ago and MAGI stays below the 15% tax bracket. Post conversion MAGI $30,000-to-$37,649. 15% tax bracket. Tax=$4,500 (i.e $1,900 more in YOUR account).