Rollover of a 401(k) is always reportable. It's trustee-to-trustee transfers of IRAs that are not reportable.
If you roll it over, you do want to do it by direct rollover (sometimes referred to as a direct transfer) to avoid mandatory 20% tax withholding.
Money in the plan of the employer from which you have separated from service plan can be taken out at any time, although distributions might be subject to early-distribution penalties depending on your age. If you roll it over to your current employer's plan, if you are under age 59½ there will be restrictions on taking distributions from the plan and, if you age 55 or over in the year that you separated from service with the old employer, you would lose the age-55 penalty exception.
To avoid limitations on the availability of funds that are presently in the old employer's plan, you might instead consider directly rolling that 401(k) over to an IRA. You would still lose the age-55 penalty exception if it would have applied (and a few other less common ones), but you would gain a few possible early-distribution penalty exceptions in the IRA. You would also have a much larger choice of investment options with an IRA.
