Improvement is great!
Yes move up your HR zones by 2bpm.
The timing of this test is such that recently you have been focused on increasing your VO2max with short duration intervals and the 20 min test is a threshold test. If you had been doing VO2max testing you would have seen a greater improvement. Next training block is a threshold block and you will see your 20 min test distance increase.
Keeping a single repeatable test over the course of training is more valuable than changing the test to make it specific for each training block.
A 2bpm HR difference over 20 mins is small. An increase or decrease in HR can mean multiple things. HR is a fuzzy pacing tool. HR is not a direct measure of exercise intensity but a response to exercise plus other variables (heat, altitude, fatigue, motivation, adrenaline, caffeine etc). The “other variables” make deciphering exactly what HR numbers mean an art form needing lots of background information.
Lynda