You can migrate. https://documentation.meraki.com/General_Administration/Licensing/Subscription_-_Licensing_Overview see the red note under Claiming.
The most important note I have is, you need to work with your partner and meraki rep on this.
I know with Co-Term you have to do it at the end of the subscription. Based on the notes on the link above I believe the same applies to PDL but there may be some back end magic they can do to assist with this as all the various end dates with a PDL could make this a drawn out process.
Perhaps someone who has done a PDL to Sub conversion will chime in further. The partner documentation I've seen has focused nearly if not exclusively on Co-Terms
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