Changing parent provider

[Fernando Ronci]

Hi,

We run our ISP and our servers are authoritative for our zone.
On July, 11th, we'll change our parent provider, which means
all our IP addresses will change too.
Considering that we are authoritative for waycom.com.ar and
NIC-AR is authoritative for com.ar, (NIC-AR is the only entity
that refers resolvers quering xxxx.com.ar to the authoritative
nameserver of the queried zone) my consult is:
How long before changing the namservers IP addresses should we
update records on NIC-AR with the new addresses?
Are there any other considerations to take into account?
We want to avoid loosing mails sent to our domain or having our
www sites unreachable.
(The TTL in our SOA record is two days)

Thank you very much,

Fernando Ronci
Waycom S.R.L.
E-mail: fernando at waycom.com.ar