I earned on Upwork in earlier years without registering. Can I still move to this setup?

Yes, but a new registration does not close the past on its own. Income declared through voluntary disclosure before any letter arrives from the tax authority carries the tax and a late payment surcharge, without a tax loss penalty. Once a letter has arrived, the cost of the same history rises. Whether the exemptions can be applied to earlier years depends on that year's own documents and money flows, so a new registration and the correction of the past are handled in a single plan.