I know this question is 5 years old now, but just in case anyone stumbles across it I would like to point out that while it certainly makes sense to "prorate" the depreciation calculation according using the mid month convention for real estate, that is specifically not allowed per the Treasury Regs. Specifically 1.168(i)-4(d)(3)(B).
If there is a change in the use of MACRS property, the applicable convention that applies to the MACRS property is the same as the convention that applied before the change in the use of the MACRS property. However, the depreciation allowance for the year of change for the MACRS property is determined without applying the applicable convention, unless the MACRS property is disposed of during the year of change.
https://www.law.cornell.edu/cfr/text/26/1.168(i)-4
Specifically, for this question when property is converted to a use with a shorter recovery period the taxpayer can either use the shorter period for the entire year OR the taxpayer can elect to ignore the change and continue to use the longer period indefinitely.
In the case of conversion to a longer recovery period, the taxpayer must use the longer period from the first day of the year.
More discussion can be found here:
https://www.thetaxadviser.com/issues/2012/oct/clinic-story-03.html
