Your understanding of the southwest change is wrong. In March your 8 credit will not convert to 9600 points. You will have the option of converting 9600 points into 8 additional credits which when combined with your current credits will yield a free ticket. You have until your current 8 credits start expiring to do this. If you don't generate 9600 points in time then your current credits will be worthless.

So what matters here is:

How much is 1 free RT worth to you

Are you going to generate 9600 points on WN elsewhere to rescue your 8 credits.

If a free ticket is worth $250 then no point in transferring SPG since those are worth more than $250 anyways.