Hi Guy Alfia
Good questions. I'll go through each in turn, but I wanted to start by saying that it will be important to stick to the deployment schedule.
1. The release schedule has been created with the size of the lead databases of our customer base in mind. It's been split alphabetically by Account String into different numbers of customers and the determining factor for how the splits were made was total database size. For example, Group 2 is only the three letters A, B and C while Group 3 has the nine letters of D through L. However, based on the lead database sizes of the customers within those groups, Group 2 and Group 3 have approximately the same number of leads in them. The schedule has been divided this way to avoid causing traffic jams in data flowing back and forth with SFDC.
Marketo continuously makes significant strides forward in our software offerings as well our hardware serving that software, such as with Project Orion that was detailed in this year's Summit. Check out the webinar located here for a description of Orion (starts at 28min in). Even with the best software and hardware out there, data can only be pushed through an API call so fast. (Whether you use a garden hose or a fire hose, it still takes time to empty a whole water tower.) The servers have set lines of communication with SFDC and all of the customers share those lines. We've created new pipelines to be devoted just to the backfill process so that it doesn't compete with the regular SFDC sync cycle, and if everyone sticks to the schedule, we know those communication lines will keep up with the traffic. But if customers don't stick to the schedule, it throws it off. (What happens if someone parks a second water tower next to yours and asks to share your one fire hose?) That's why we ask everyone to stick to the schedule. We know this schedule may not work for everyone, so you're welcome Contact Marketo Support to request a schedule change. We'll be happy to accommodate as best we can.
2. The old fields being replaced can be deleted by uninstalling the MLM package. The details of cleaning up SFDC can be found here and here are SFDC's instructions for Uninstalling a Package. One thing to note is that if you have any reports or workflows in SFDC that reference these fields, SFDC won't let you uninstall it until those dependencies have been removed. Once data stops flowing to the old fields, they'll be obsolete, so there's no real need to keep them.
3. The old fields and the MLM package can be removed as soon as the the fields have been created. Just be sure to update your workflows and SFDC reports if you have any to connect to the new fields like noted above. All of the details can be found in our Changes to Marketo Salesforce Sync – Under The Hood documentation.
4. The fields do need to be recreated as new fields. The configuration change will shut off communication to the existing fields and new fields must be created in SFDC to replace them. Changing field names on the existing fields won't work unfortunately. You can find the info on keeping field data in SFDC in the documentation for Adding Marketo Fields to Salesforce here.
5. The fields currently exist in SFDC on the Lead and Contact objects already, and it's strongly recommended that you recreate them on both the Lead and Contact objects. It isn't required. You can do it on just the Lead object, but as it says in our FAQ doc, it's best to create them on both the Lead and Contact as well.
I hope this helps! Let me know if you have any other questions.
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