There are a large number of instructional books out there on obtaining an Instrument Rating in many languages and you need to do some serious reading on IFR flight plans and clearances.
- Go to your local pilot shop usually located at an airport with private airplanes or ask a local flight instructor to recommend a book.
- Read the chapters on filing flight plans, obtaining clearances, adhering to clearances and lost communications procedures.
You will learn as has been said repeatedly by every IFR pilot on this thread that:
- A flight plan proposes a route with requested altitudes at a requested speed at a requested time
- A clearance is ATC's best effort to accommodate your proposal without conflicting with other traffic.
- You DO NOT deviate from your clearance without permission from ATC except in an emergency.
As well, there's a good probability the Legacy pilots used a commercial provider to generate their flight plan by computer. Computers can be very particular about adhering to quadrantial flight levels when generating flight plans when ATC will normally prefer to simply assign a single cruising flight level.