Revman, teams were allowed a certain number of "jokers" to spend on updating homologated parts (it's not free-for-all like F1 for example), split between engine and chassis. How they count each update is a bit open to debate, so one joker could be a single new hole on the OEM chassis or 1 bodywork and all other panels that connect to it... So for you it would be a bit of work (but fun) trying to figure out who used the jokers on what without looking at the homologation dossier. It's a "use it or lose it" situation so one can assume every team used their full allowance (or maybe not??). For 2018 there are less jokers available, so they must be more selective about what is worth upgrading this way. Then there are parts which are homologated but multiple options are allows and can be added through the year (every 3 months) without restriction. The distinction between the two types of homologation would not be obvious without reading the technical regulation, unfortunately.

So what it all means is, some cars may look obviously different externally, while some may not, but you bet all of them would have had upgrades throughout 2017 and between the seasons for parts where jokers were not needed. I think most cars had one or two clear deficiencies, which would have been the focus of upgrades. We will now wait for Monte Carlo 2018 to see how well they improved them.