Generally, you can make changes in your 401k plan as often as you want; I use the word generally because employers can impose their own restrictions to prevent employees from trading in their 401k plans. ACap strongly advises against actively trading in your 401k or try to time the markets. Rather, rebalance your portfolio on a periodic basis to minimize risk. Review your 401k at least quarterly to ensure the allocations you initially selected have not deviated from your intended percentages; if they have, then rebalance your entire portfolio (including 401k) to bring it back in line with your goals.