Vou considerar que você trocou apenas a flash, a DRAM continua a mesma correto?
O IOS fica na flash, portanto se você efetuou esta troca, será necessário colocar o IOS na flash nova.
1 - Segue um procedimento para atualizar o IOS utilizando o tftpdnld:
rommon> IP_ADDRESS=10.99.18.11
rommon> IP_SUBNET_MASK=255.255.255.0
rommon> DEFAULT_GATEWAY=10.99.18.1
rommon> TFTP_SERVER=10.99.18.15
rommon> TFTP_FILE=c2801-entbase-mz.123-14.T6.bin
rommon> tftpdnld
Invoke this command for disaster recovery only.
WARNING: all existing data in all partitions on flash will be lost!
Do you wish to continue? y/n: [n]: y
Receiving nome_do_arquivo_ios.bin from 10.99.18.18 !!!!!!!!!!!!!!!!
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!!!!
File reception completed.
Copying file nome_do_arquivo_ios.bin to flash
Erasing flash at 0x607f0000
Programming location 603a0000
Rommon> confreg 0x2142
Rommon> reset
Mauricio Nakano
msn: mauriguinakano@hotmail.com