Starting with exports from both the new membership system and Zoho CRM Credible analysed the data and constructed 6 separate scenarios for each user record - some being subject to more than one scenario.
With these scenarios understood, we built automation flows in Zapier that dealt with some of the simpler outlying cases (e.g a brand new member or a membership that had expired). We updated the CRM system using the results spreadsheets from our analysis and some automation using Zapier and checked against our analysis.
The bulk of the work was in connecting the new membership database in Zapier to Zoho CRM using a combination of checks, loops and queries to account for all possibilities. E.g If a user has changed employers, much of their CRM record will remain the same but the email and company values will change. Companies in Zoho CRM are a parent record of the Contact, so we had to automate a check to search for the new employer name, and create it if not found before updating the Contact record with the new relationship.