Yes you can. If you change flavors, the new flavor may pick up the old one a bit depending on which is stronger. After a couple of refills, the old flavor may disappear, but again, it depends on what it was. You will never get rid of a menthol, there will always be a hint of it.
As far as the nic, if you are filling with a lower nic, the first fill may be slightly higher. For example, if you had 12mg in there and filled with 6mg before it was completely empty, you may have 7 or 8mg in there after combining. I do that all the time
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