"......do they then allow bottom interval to reflow into...

"......do they then allow bottom interval to reflow into well."

From my knowlege and understanding the simple answer is NO!

In order to test a higher layer/zone, if they decided that this present level was uneconomic, then they must cement that lower layer to prevent any escape of any gas or liquids from that lowest zone.

Once they abandon that lowest zone and cement it off there is basically no going back.

That is mainly why they would be reluctant to abandon the lowest layer they are presently testing if it proves to be commercial. (A bird in the hand is worth two in the bush thinking)

If the level you have tested is commercial then take it. Don't gamble on thinking the next layer up is going to be better. Get the cashflow.
You can always find out what is there at a higher level by drilling another well or, alternatively, checking it out once your lowest producing layer ceases to be productive.

You don't gamble. The stakes are too high.