@tomleslie  It's really weird, I tried to start up an entirely new document,  and do  all of the calulaltions once more, and this time it evaluatede to 
2.114163508*10^7 [kg/s^2] 

Exactly as before but the only difference beeing that this time I was able to directly  replace the unit as intended. 
Before creating a new document I had tried to apply the restart within the document several times. 

Thanks for your answer and time though I'm even more confused right now. I don't see how I can do everything the excact same way with the excact same values and units (triple checked), and end up with the excact same result, but now out of nowhere there is no problem with replacing of the unit. Thank you Anyway!

Best regards
Anders