@HenryGe - No, you do not need to convert everything. existing subscriptions will stay as S- for the time being. New subscriptions being created will have I- so you will need to support both formats.
The only logic you need to code for is in the subscr_payment IPN, the transaction_subject variable will work like this:
in case of S-: transaction_subject has 'custom_code' value
in case of I- : transaction_subject has the 'item_name' related to the subscription
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