No, it is not possible to upgrade the base variant of Creta to SX variant.
The base variant of Creta diesel is equipped with 1.4 litre engine whereas Creta SX diesel comes with 1.6-litre engine. Hence it cannot be upgraded. Coming to the petrol variant, both the variants are equipped with 1.6 litre engine but there is a vast difference in the extra features available in SX variant. So, we recommend you to purchase the SX variant- if possible, the upgrade would cost you more.