The conversion from the traditional IRA to the Roth IRA is a taxable event. The entire amount is taxable as ordinary income unless you have some amount of nondeductible contributions in your traditional IRAs. In addition to any nondeductible contributions you made directly to the traditional IRA, you could have nondeductible contributions that resulted from a rollover of after-tax money from the 401(k), although this is not common. An after-tax amount rolled over from the 401(k) would generally have been reported in box 5 of the Form 1099-R that reports the rollover from the 401(k) to the traditional IRA.
If the tax result of the Roth conversion is not to your liking, you can undo some or all of the conversion by asking your Roth IRA custodian to do a recharacterization back to a traditional IRA of some or all of the amount converted. You have until the due date of your tax return to do a recharacterization.
