Syncing on top of old chain ... works:
gsoil
INFO [05-12|15:42:48] Starting peer-to-peer node instance=Gsoil/v1.6.1-MMF-6ccff9a9/linux-amd64/go1.8
INFO [05-12|15:42:48] Allocated cache and file handles database=~/.soil/chaindata cache=128 handles=1024
WARN [05-12|15:42:49] Upgrading chain database to use sequential keys
WARN [05-12|15:42:49] Found genesis block without chain config
INFO [05-12|15:42:49] Initialised chain configuration config="{ChainID: 1337 Homestead: 0 EIP150: 0 EIP155: 0 EIP158: 0 Engine: ethash}"
INFO [05-12|15:42:49] Disk storage enabled for ethash caches dir=~/.soil/gsoil/ethash count=3
INFO [05-12|15:42:49] Disk storage enabled for ethash DAGs dir=~/.ethash count=2
INFO [05-12|15:42:49] Initialising Soil protocol versions="[63 62]" network=42
INFO [05-12|15:42:49] Loaded most recent local header number=2000267 hash=8b4f6b
2881fd td=32538335992977008
INFO [05-12|15:42:49] Loaded most recent local full block number=2000267 hash=8b4f6b
2881fd td=32538335992977008
INFO [05-12|15:42:49] Loaded most recent local fast block number=2000267 hash=8b4f6b
2881fd td=32538335992977008
WARN [05-12|15:42:49] Blockchain not empty, fast sync disabled
INFO [05-12|15:42:49] Starting P2P networking
INFO [05-12|15:42:51] RLPx listener up self=enode://46fce6fdc328596df97c8c2e4d1744b9bf5bfd540 ff8cf95e4880d797bd92e2cb52a81f38a9a49e51079f875c1adbdc2d4d6f503253e2d7188a494d421bd968b@[::]:30403
INFO [05-12|15:42:51] IPC endpoint opened: ~/.soil/gsoil.ipc
...
So ... looks like updating AFTER a new version has already passed the fork-block ... is unproblematic.
config="{ChainID: 1337 Homestead: 0 EIP150: 0 EIP155: 0 EIP158: 0 Engine: ethash}"Please make a full resync to load the new config. By default fast sync is enabled - so it will take you only a couple of minutes.
config="{ChainID: 42 Homestead: 2000000 EIP150: 2463000 EIP155: 2675000 EIP158: 2675000 Engine: ethash}"