To Refi or Not to Refi..that is the question

[Grant Cleveland]

I have a mortgage I owe 280,000 on at 4.25% 30 year fixed with 27 years remaining.  

Down the line in let's say 1-3 years there's a chance I'll move and would like the option of converting this to a rental.  

I can rent for 2,100 (conservative).  Assuming costs of property management at 10%, repairs at 5% and vacancy at 8% I would have negative cash flow -250 or if I self manage -40.

I have a crazy notion to throw 42,400 (loose money lying around in savings) at the mortgage/fees and refinance to 3.75%.  This would drop the monthly payment to enough to have a positive cash flow +150 or +360 if I self mange.  

What factors need to be taken into consideration before making the decision?  

Thanks!  -Grant