How Change route object origin entry

Hi

A question 

I need to change the "Origin" entry on a /20 route object. This route is to
be advertised via a different upstream provider, from a different AS number.

The upstream provider will not advertise the route "until RIPE have been
informed".

I cannot change the route object entry. I get the following "Error:
Hierarchical authorisation failed, request forwarded to maintainer" and I
get the request forwarded to me.
The mnt-by on the route object is not the problem - I can update other
objects with that mnt-by object and I can change the "notify" entry on my
route object.

Therefore the error must be because of the mnt-by entry within the
"receiving" AS number. (which I am not authorised to update) but I don't
understand how. 

Is this the correct diagnosis? and is there something else I should be
doing?

I could ask RIPE hostmaster to make the changes but the ticketing system
could take a couple of weeks.

I could ask the new upstream AS owner  to add me to their mnt-by entry but I
am nothing to do with them.

All suggestions gratefully received.

kevin