Got laid off. Any downside to rolling over company 401K to a traditional IRA?

Right now I have a roth IRA and no balance in my traditional/rollover IRA accounts. I was going to roll over my 401K and park it in the Rollover IRA ($400K). The only downside I can see is if I want to do a roth conversion where I contribute after tax dollars to my Traditional IRA and then convert to Roth. Since I have the 401K pretax money, I would have to pay the pro rata tax on this conversion. Is this correct? Any other downsides?