I have had this happen a couple of times because of splits or sales of portions of the company.
The general timeline was to announce how the split was to be handled; then the split; then a freeze in purchasing stock in the other company; then a freeze in sales; followed by a short blackout period; then the final transfers to funds/options/cash based on a mapping announced at the start of the process.
You need to answer two questions:
- Is the default final mapping of your investment what you want to do?
- Do you want to wait until the deadline to either take the default option or make your own investment choice?
To determine if the final transactions will make the market move you have to understand how many shares are involved compared to the typical daily volume. There are two caveats: professional investors will be aware of the transaction date and can either ignore the employee transactions or try and take advantage of them; There may also be a mirroring set of transactions if the people left in the old company were awarded shares in your company as part of the sale.
If you are happy with the default mapping then you can do nothing, and let the transaction happen based on the announced timeline. It is easy, and you don't have to worry about deadlines.
If you don't like the default mapping then you need to know when the blackout period starts, so you don't end up not being able to perform the steps you want when you want.
Timing is up to you. If the market doesn't like the acquisition/split it make make sense to make the move now, or wait until the last possible day depending on which part they don't like. Only you can answer that question.