Can I change by upper-grade-license edition after buying lower-grade-license edition, e.g. from Professional edition to Ultimate edition? If I can, how much money I must pay?
Yes, it is possible. You will pay only difference between prices of these two editions. (The 1-year period for updates for new versions will be preserved from the original license. The prolongation of update license for new versions by one year costs only 50% of original prices.)