This is perhaps the most critical element of your helpdesk migration - timing. Unlike a CRM migration, where you can just halt data-entry over a weekend while the data loads, your helpdesk is an active system. Customers can send in ticket submissions anytime, day or night, and they expect timely support. Therefore it is critical that you have a gameplan on how to switch your helpdesk to Zendesk while minimizing downtime, for both your customers and your agents who need to serve them.
Sometimes it's best to look at the project from the end, and work your way backwards.
Look first at how long the actual migration will take.
This means that once you "turn off" your old helpdesk system and route all communication to Zendesk, how long will it take to move all your users and ticket history over?
The answer here is not a one-size-fits-all. The length of your helpdesk migration depends on a combination of factors, including the speeds of both Zendesk and your old helpdesk, as well as the number of records to migrate.
Every helpdesk system has an API speed, which essentially just means the number of records that can be moved over a period of time (for example, per minute, per hour, per day). Some helpdesks are really fast, but others have throttled speeds that result in the data moving very slowly. On average, importing data into Zendesk moves at a speed of about 50,000 tickets per day (24 hours).
Review how much time you need for your preparation steps.
Once you know how long it will take to execute the migration, take another step backwards and assess how long you will need to route all your communication channels into Zendesk. If you want the migration to start at 5pm on a Friday, you should allow yourself enough time before that deadline to route all potential sources of customer communication into Zendesk. This way, once the migration starts at 5pm, there is no chance that a new ticket will be created in your old helpdesk.
Along the same lines, you also need to communicate to your support team when their final cut-off time is to work on tickets in your old helpdesk. Just as you don't want any new ticket submissions there once the migration starts, you do not want any updates being made to existing ticket conversations there either, as this would risk not be reflected in the data that is brought over to Zendesk.
These preparation steps will require your attention in the days leading to, and on the day of the migration. It's also imperative that you have the time and attention during the actual migration to be present for any issues, updates, etc. that arise. Keep this in mind when you are scheduling the start of your final migration, particularly if you are looking to migrate over a weekend.
Make time for testing.
Once you know your day-of timing, you can then work backwards to determine how much time you want to dedicate to testing prior to the switch.
This is a personal decision - some companies want to move quickly and others dedicate months to this process. It's up to you, but no matter what, you need to have some buffer time planned into your project to test and validate the import results before you perform the live migration.
Make sure that whoever is performing the migration for you is able to accommodate your schedule as well. If you are expecting a tight turnaround testing phase, you have to be able to count on your migration partner to meet this deadline as well. Otherwise, it can throw off the rest of your plan to cutover to Zendesk on time.