The game has changed but highsec missions, mining, PI, etc will remain familiar unless you’ve been gone a really long time!
I recommend you login your existing character and play as an Alpha for a bit - it’s free. Run a set of career agents, there are 3 sets for each race https://wiki.eveuniversity.org/Career_Agents. You will be locked to T1 ships of your birth race and a limited skillset while playing in the Alpha state. Then run the SOE Epic Arc https://wiki.eveuniversity.org/The_Blood-Stained_Stars to help bring back old skills.
You can still purchase PLEX from CCP and sell it in game for ISK. It’s more granular now they did a 500-1 split earlier this year.
In addition to what Do Little said; because of the PLEX changes $20 USD worth of PLEX (500) is now worth more in game.
If it has been a long time and you feel rusty, I would recommend making a new character either on your current account or on a disposable alpha account. We have a new tutorial that covers a lot more of the game than any tutorial before it. This may help you get going.