In-Service Withdrawal Window?

[Alf]

To get rid of a few illiquid nonstandard assets, we want to allow in-service withdrawals (age 59 1/2) and will open it up to all participants (not just self-directed), but we don't want it to be permanent (paternalistic to keep retirement savings intact) and don't want to have to grandfather the right for ever.

Can we just offer it during a certain pre-specified window and avoid grandfathering?? How long does the window have to be (one day is enough to get the nonstandard assets out, but one full year sounds less abusive)?? Does it matter if only HCEs end up using it, or will 401(a)(4) be ok as long as it is available to all??