Download packages first then upgrade.

http://www.howtoforge.com/how-to-upgrade-debian-squeeze-to-wheezy#comment-33774

I prefer to download first
Submitted by Grant (not registered) on Wed, 2013-05-08 22:04.
I prefer to download first, before installing any updates, making it 5 steps:

Update the sources

apt-get update

apt-get dist-upgrade -d

apt-get upgrade

apt-get dist-upgrade

I have had issues in the past with a loss of network connectivity mid update causing serious issues.

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I’ll try above method definitely. It makes sense. Bad network connecting during the upgrade process causes some time irreversible problems.