Welcome to ECF,
Warm water will loosen almost any tank, the only issue is that materials expand at different rates so you would have to bring the atomizer up slowly to avoid thermal stress. Once its warm then it should be easy to remove any stuck atomizer without any issues.
Trust me, I learned the hard way about thermal expansion after an aspire tank shattered on me once and then after a replacement was secured I did the same thing (locked down the top cap, again) and tried the method above, it worked no problem.