Problem with Altering a table column

Version 9.5
OS Centos 6 running on a VM
I have a large table (over 100Gb including indexes) 
The machine is slow
I have issued and alter column against the following;
 price_id         | integer                     | not null default nextval('bet.price_price_id_seq'::regclass) | plain   |              |

As;
ALTER TABLE bet.price ALTER COLUMN price_id TYPE bigint;

The process has taken a very long time, I know that it is rebuilding the table and a lot of temp files have been created, deleted etc
It now seems to have gone quiet (no temp disk files etc), it still seems to be clicking up CPU
- Is there a final part of the process where it may just be doing validation etc?
- Is it safe to terminate the process?
Many thanks

Doug Reed dougreed765(at)yahoo(dot)com 07973-132664https://uk.linkedin.com/pub/douglas-reed/33/326/2b