1.No such restrictions as such AFAIK on no.of times, as such provided CNF /RAC accommodation is available for the proposed date.
2.No such series of trains for which such change of date cannot be exercised.
3.Reservation charges are applicable and it also depends on the time of change from date of journey of original ticket.
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You...
more... can see set of rules here:
click here...
Only PRS tickets can exercise this option, E-Tickets cannot be modified...More or less can say instead of going thru all these its as good as cancel the existing one & book a fresh ticket for the proposed new DOJ.