You should check which version you are using as a base. If your upgrade is from Kx-Ultimate (can be K8-U ..... K12-U), then the 50% price is ~300€ (298 in Thomann). It is worth it.
If your base version is K2....K12 (not ultimate), then the price is considerably higher, the procedure is not so attractive at 458€, it would be better to just purchase the K12U-CE to K13U-CE Update (about 500€ everywhere). You'd save 41€ in the case of Thomann. Ah, well, in this case it might be worth it, you could resell your Kx to K12UCE upgrade at a better price than KxU to K12UCE.