Switching rent payment date from 1st to 15th

[Arey Hiel]

Tenant has been paying on month to month basis on 1st of every month.  However, tenant wants to switch paying rent on 15th of every month. Should I prorate first 14 days of rent in next months rent when tenant wants to pay from 15th onwards? 

So when tenant pays rent ($1000) on July 15, should I ask him to pay pro rated rent for July1 -July 14 as well?