I do not want to depreciate capital improvements I made to an investment property (house) that I planned on selling, but after six months on the market, I now plan on renting. Is it mandatory to depreciate the capital improvements once it is put up for rent or can I add the cost of the improvements to my purchase price as a new basis when I finally decide to sell?

You must use Form 4562 (to report depreciation) beginning in the year your rental property is first placed in service, and beginning in any year you make an improvement or add furnishings.
Read IRS Publication 527: Residential Rental Property; Limits on Rental Losses