You can't know until the very final day of your league, but what happens is as follows (using division II to III in West Indies for ease of explanation):
Teams that are automatically relegated and promoted swap places with the equally ranked in the upper division
Teams that play qualifying matches are seeded against each other, highest v lowest.
Not too easy to see without examples, so, taking the current tables from Division II in West Indies, that means:
Division II, play-offs:
Banagher Knight 28 (+16.40) II.2
Atlanta Mosquitoes 22 (-3.34) II.4
Doctors 20 (+8.49) II.3
Tythe Barn 20 (+2.21) II.1
Twelfth Men 18 (-5.35) II.4
Indy 12 (+0.88) II.3
Lazers 12 (-15.69) II.1
Sandtigers 12 (-17.76) II.2
Division II, relegated teams:
The Flintstones 12 (-15.90) II.1
AF '81 12 (-19.30) II.2
Itsy Bitsy Spiders 8 (-13.43) II.4
Metairie Cricket Club 8 (-33.73) II.3
Sangre Grande CC 0 (-61.35) II.3
Salibea CC 0 (-61.72) II.1
Parham CC 0 (-62.57) II.4
Five Islands CC 0 (-68.74) II.2
Division III, promoted teams:
1 Avalon Angels III.14 48 56.44
2 Shadoedale CC III.8 48 54.36
3 Trespassers W III.12 48 49.62
4 Norse Athletic Club III.10 48 48.70
5 Mud poodles III.4 48 44.37
6 Blue Hoops CC III.11 48 39.69
7 Batty Men III.5 48 33.83
8 New Courland III.6 48 32.91
Division III, play-off teams:
9 Bloody Americans III.2 48 19.85
10 Ackee an Saltfish III.15 46 34.01
11 Strong Arm Steady III.7 44 48.32
12 Crouching Tigers III.16 44 44.73
13 Las Vegas Wildfire III.13 44 42.89
14 Canton CC III.9 44 42.26
15 Groovin Ganjamon III.1 44 40.29
16 St. Lucia Wanderers III.3 44 28.49
Play-off matches would thus be:
Banagher Knight (1 division II) v St. Lucia Wanderers (16 division III)
Atlanta Mosquitoes v Groovin Ganjamon
Doctors v Canton CC
Tythe Barn v Las Vegas Wildfire
etc.
Sandtigers v Bloody Americans
While the teams to swap places would be something like:
Avalon Angels (III.14, 1 division III) swaps with The Flintstones (II.1, 9 division II)
Shadoedale CC (III.8, 2 division III) swaps with AF '81 (II.2, 10 division II)
Trespassers W (III.12, 3 division III) swaps with Itsy Bitsy Spiders (II.4, 11 division II)
etc.
New Courland (III.6, 8 division III) swaps with Five Islands CC (II.2, 16 division II)
This is of course very close on net run rate, so hard to calculate before the final day which division you'll end up in.