401(k) is a long term investment for retirement.  The question of transfering money to bond should be looked this way.  It should be an asset allocation question based on your age.  As you are closer to the retirement age, you should have allocated more asset to bond.  The allocation that I will use is 70% equity / 30% bond when you are in your 40s, 60/40 when in 50s, and 50/50 in 60s.  But these allocations are just a general guideline, you can hold more equity than bond if you have more risk tolarence.  

If you think that the equity market is too high and would like to move some money to bond, sure, you can do that.  Since we are talking about 401(k), I would think that most 401(k) plans only have the choises for bond funds.  In current interest rate normalization period, you need to be careful of choosing bond funds.  You need to look at the fund's duration.  If a bond fund has the duration of 5 years, that means when interest rate goes up 1%, the bond fund will lose about 5% of asset value.  If that bond fund yield is 2%, then the total return will be negative 3%.  The best way to do the allocation is to find a bond fund which has a shorter duration, maybe around 1 to 2 years.