The Percent Full Threshold value of the storage pool (70% is the default) simply defines when the system will trigger the first alert.  It doesn't mean that the pool won't continue to allocate as necessary (up to 100% which you of course want to avoid).  Keep in mind that the system will always send an alert when it reaches 85% regardless of what is set and this is also why you can only modify this value (up to and including) 84%.  I believe this is the confusion that prompted your questions.  Knowing now what that parameter defines, you can now see that yes you can safely modify that value without disruption to data.
I have a VNX7500 configured with only Storage pool (TP1). And now I can see it’s about to reach the Pool threshold limit (70%).
VNX Storage Pool threshold limit.