Having gone though this type of event a few times it won't be a problem.
On a specific date they will freeze your accounts. Then they will transfer the funds from custodian X to custodian Y. It should only take a day or two, and they will work it around the paydays so that by the time the next paycheck is released everything is established in the new custodian.
Long before the switch over they will announce the investment options in the new company. They will provide descriptions of the options, and a default mapping: S&P 500 old company to S&P 500 new company, International fund old company to international fund new company...
If you do nothing then on the switchover they will execute the mapped switches. If you want to take this an an opportunity to rebalance, you can make the changes to the funds you invest in prior to the switch or after the switch.
How you contributions are invested will follow the same mapping rules, but the percentage of income won't change. Again you can change how you want to invest your contributions or matching funds by altering the contribution forms, but if you don't do anything they will just follow the mapping procedures they have defined.
Loans terms shouldn't change.
Company stock will not be impacted.
The only hiccup that I would worry about is if the old custodian had a way for you to transfer funds into any fund in their family, or to purchase any individual stock. The question would be does the new custodian have the same options.
If you have more questions ask HR or look on the company benefits website. All your funds will be moved to the new company, and none of these transfers will be a taxable event.
Edit February 2014: based on this question: What are the laws or rules on 401(k) loans and switching providers? I reviewed the documents for the most recent change (February 2014). The documents from the employer and the new 401K company say: there are no changes to the loan balances, terms, and payment amounts. Although there is a 2 week window when no new loans can be created. All employees received notice 60 days prior to the switchover regarding new investments options, blackout periods.