There is what is known as a "corporate action" occurring with your company stock so yes, it will be "locked up" until they finish doing whatever they are going to do with it.
You may end up with shares in the new company or $.
Example: I held shares of WNR which was acquired by TSO; the tender offer was for $ (the better deal), an exchange of shares or a combination of both. I requested the $ and am now waiting for the process to conclude. As of now, the (value of my) WNR shares don't even show in my brokerage account. Eventually, $ or $+some TSO will show up.
When you say old 401(k) into existing 401(k), do you mean a 401(k) from a previous employer which you wish to roll over to the current (sold company) employer or ?
If you're talking about rolling your $ away from your current (sold company) employer that may be possible since you technically no longer work for said company (once sold). Otherwise, you can't touch a 401(k) until you are no longer employed by the employer through which you are contributing.
With all of the (possible) turmoil, I would stand pat, at least for now.