Well it has been over a year since the last time I broke my database, but I did it again today!  This time I increased the sga size without also increasing some other parameter, and the database would not start, and since it would not start, I could not change the parameter.  What to do?

After a bit of hacking the SPFILE to no avail, I worked out the following process:

First, log on to Oracle as an administrator:

sqlplus / as sysdba

Next, create a nice editable plain text PFILE from the “binary” SPFILE:

create pfile from spfile;
exit

Now you can edit PFILE to change whatever parameters you need to.  Once you are done, you can start up the database from the PFILE:

sqlplus /  as sysdba
startup pfile=initSID.ora

Problem solved!