Ok, here's my attempt at a template for calculating these changes:
Starter to Reliever Change Google Doc
I think it's read only right now, people should be able to download it still. I can always make an editable version as well.
Here's how it works:
1) Enter player's current stats for IP/H/HR/K/BB in current section (Row 3, Columns F-J)
2) The sheet will calculate current BABIP, K/PA, and HR rate in Column J.
3) Sheet will also calculate targeted values for BABIP, K/PA, and HR rate in Column K.
4) In the new section (Row 3, Columns L-P), copy over the pitcher's starting stats. Then change H, HR, and K until new values for BABIP, K/PA, and HR rate match targeted values.
5) The spreadsheet calculates the /9 values based on the new stats.
6) Find corresponding stats in ratings table below and those should be new in-game ratings for H/9, K/9, and HR/9.
Please feel free to offer feedback, would love for someone to take a look at the calculations I'm using to see if they seem logical.