When I reallocate the money in my 401k, am I paying hidden fees?

When I reallocate the money in my 401k, am I paying hidden fees?

I've read about management expenses, but not sure if they are taking money out when I move money out of one fund to another

There are expenses associated with the transfer, but they are not directly paid by you. They are paid by the company, which should absorb them. Actually, every 401k investor should review his asset allocation periodically and make certain that the assets are allocated to provide good diversity and also it is suggested by the professionals that ones porfolio be rebalanced periodically to improve overall returns. The idea being that if one of your assets has increased significantly in value some should be sold and the money reinvested in those that have not performed so well. Sort of buy low, sell high.

Usually not. They are supposed to tell you the exact charges you pay.
However when you move funds you are actually selling stocks (meaning shares)in one fund and buying stock or shares in another company.
For example if you have been investing in Fund xyz for 10 years and it has rizen from $3 a share to $22 a share and you onw 10,000 shares your value is 220000. I fit started to slide down and you moved the funs at 14 all of the shares you brought and paid over 14 for you would lose money on. If your average share price for the last 5 years was 19 a share than you averaged 5 bucks a share loss.
If you moved the funds at 20 after it started to slide and dropped your money into a safety principal fund and the old fund to 8 bucks a share and you brought back into it at that price and it again climbed to 22 you made a killing.
When you lose money by moving it is when you paid more for the fund than it is selling . Sometimes as was our case at work the company moves it for you when your 401-k carrier changes hands or is brought out. Some companies do not charge a fund transfer fee but most do. usually they ones which don't are companies which do not turn much and pay your brokers themselves for sending your business to them.

As a general rule the answer is no. All plans are different, but I haven't seen it.