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in this instance I am assuming that the BL1 ia a previous updated schedule not the true unprogressed basline

of course you can do it graphically showing the progress on the baseline bar vs the current bar.

The only way that I can see doing it in the data table as value is to create 2 udfs (user defined fields)

one called Physcial % Complete Gained and another called Last month Physcial % Complete

Then you would have export the Physcial % Complete from the baseline into excel and then import that into the current schedule as Last Month Physcial % Complete Value 

Then run a global change 

Where Physcial % complete is greater that 0%

then Physical % Complete Gained = Physical % Complete  -  Last month Physcial % Complete