Sell back individual upgrades

I thought of this the other day when the new upgrades came out. How about an option to sell back upgrades for 50% of the SP back so you can put those points toward newer upgrades? Perhaps make it cost protocoins instead, so maybe 20 protocoins to sell an upgrade back. This way users who bought an upgrade, then many days or weeks later realise they never use it, can have the opportunity to cash it in for some value back to put towards a different upgrade they may want or need.
Thoughts?

Good idea