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Because deciding what to leave takes judgement and importing everything takes an afternoon. So eleven years of dead contacts, duplicate companies and deals that never existed come across.
The new system is then distrusted from week one, for exactly the reasons the old one was.
Open deals, active customers and their contacts, the last two years of closed-won for reporting, and the products or rate cards you actually sell.
Bring closed-lost only if you genuinely analyse it. Most teams do not, and it doubles the data for nothing.
Only if you actually analyse them.
Yes, by batch, which is why they should be batched.
About a month, with the old one read-only.
Half an hour with your own data usually saves reading three of these. The guides will still be here afterwards.