Sorry for not understanding your question exactly.
An MSAN (and its backhaul) be will brought into service before any subs (end users) are connected to it.
Any DSLAM, as you say, won't be taken out of service until it's last sub has gone, whether that's all at once in an overnight cut-over or in dribs and drabs over a period of time.
Backhaul is (a lot) more than just a fibre patch lead.
Even if they are physically compatible, you couldn't simply disconnect the SDH STM fibre interface of a DSLAM and re-patch the same lead into an MSAN and expect it to work without a significant amount of network re-configuration.
As you probably already know, there's a (hopefully) short period of down time, when an end user's copper pair is re-jumpered over from a DSLAM to a C-MSAN.
As it's more cost effective to migrate end users in bulk, overnight, many users could experience service interruption on the same night for that reason.
Is that any better?