OK - if adjusting Numbering Scheme is unavoidable then here's what I would do...
Use the Database Shell to create a number of sequences for then next number of years... 10, 20 50 100 more (your call) someone a hundred years from now (or at whatever point) will need to remember to create additional sequences. If I recall correctly this is done with SQLPlus and it is documented in the Help Center. You need to have the permissions to get into the database shell to do this.
Then adjust the Object Initialization Rule (below puts a 15 as a prefix to the numbers, sets 4 digits and pads with 0) . You set the starting number when you create the sequence. This would result in 15000N
Then each year change the OIR. Change the prefix in the first Arg, and the sequence in the second.
Note - I have made up the name of the sequence, so you will use the sequence name(s) that you create.