assuming you just want to add a new entry at the end of an acl, you don't really need to unapply and remove the entire acl.
e.g.
router(config)#do sh run | in access-list 155
access-list 155 permit icmp any any
access-list 155 permit esp any any
router(config)#access-list 155 permit gre any any
EGMNSWSYDR01(config)#do sh run | in access-list 155
access-list 155 permit icmp any any
access-list 155 permit esp any any
access-list 155 permit gre any any
however, as you already know, acl works in order. in other words, if there is a deny statement has a higher order in the acl, you may need to unapply and remote the entire acl in order to make changes.
an alternative is to use extended acl, which you may be specify each entry with the sequence number.
e.g.
router(config-ext-nacl)#do sh access-l test
Extended IP access list test
10 permit icmp any any
30 permit gre any any
router(config-ext-nacl)#20 permit esp any any
router(config-ext-nacl)#do sh access-l test
Extended IP access list test
10 permit icmp any any
20 permit esp any any
30 permit gre any any
