I haven't tried it. But think you can do this as follows.

Code:
1. Create a new initSID.ora file using the current spfile.

2. Start your instance using the initSID.ora file

3. Create the new spfile on the new location

4. Now rename the old spfile on the default location to spfileSID.old
   Edit the spfile location in the initSID.ora file to point to the
   new SPFILE

5. Try starting the instance without specifying the any parameters.
   I think the system would fetch the spfile from the new location
   and start the instance.
This would require a couple of shutdown and startup on the instance.

Sam