You don't pick up where you left off for depreciation.  You restart the 27.5 years anew, using the lower of (a) Adjusted Basis or (b) FMV on conversion to a rental. 

The Adjusted Basis is likely (1) original purchase price (including certain closing costs), minus (2) depreciation allowed/allowable, plus (3) cost of improvements.

Make a note of the prior depreciation allowed/allowable for whenever the property is sold, because that will be factored into things.