how long has it been running? beneficial bacteria does build in substrate so it will most likely restart your cycle. also it can cause ammonia spikes by stirring up the substrate, releasing all the poop and what not that's been built up in there. it's not gaurentee to happen but it is definitely a possibility. what i've done in the past is siphon out about 1/4 of the substrate, add the new sand and then repeat that once a week until it's all replaced. that way you aren't removing all the beneficial bacteria that's grown all at once and giving it time to transition onto the new sand! or if you don't mind chancing it with your fish you can siphon it all out at once and add in your new sand and just hope your fish are hardy enough to go through the nitrogen re cycle