Sorry in advance if I'm confusing. I don't really know the exact terms to use.
I have made maps in 39f before and after you embarked you could move that big yellow X on the grand overview screen anywhere but to the last tiles on the map. So if the map was like
MMMMM
MOOOM
MOOOM
MOOOM
MMMMM
You could move it to any O but not to the Ms. I just generated a map using this
[WORLD_GEN]
[TITLE:old times]
[SEED:2660773182]
[HISTORY_SEED:1849857474]
[NAME_SEED:524072702]
[DIM:129:129]
[END_YEAR:1050]
[BEAST_END_YEAR:200:80]
[REVEAL_ALL_HISTORY:1]
[CULL_HISTORICAL_FIGURES:0]
[ELEVATION:1:400:401:401]
[RAINFALL:0:100:200:200]
[TEMPERATURE:25:75:200:200]
[DRAINAGE:0:100:200:200]
[VOLCANISM:20:100:200:200]
[SAVAGERY:0:100:200:200]
[ELEVATION_FREQUENCY:1:1:1:1:1:1]
[RAIN_FREQUENCY:1:1:1:1:1:1]
[DRAINAGE_FREQUENCY:1:1:1:1:1:1]
[TEMPERATURE_FREQUENCY:1:1:1:1:1:1]
[SAVAGERY_FREQUENCY:1:1:1:1:1:1]
[VOLCANISM_FREQUENCY:1:1:1:1:1:1]
[GOOD_SQ_COUNTS:25:251:503]
[EVIL_SQ_COUNTS:25:251:503]
[PEAK_NUMBER_MIN:12]
[OCEAN_EDGE_MIN:1]
[VOLCANO_MIN:100]
[REGION_COUNTS:SWAMP:260:1:1]
[REGION_COUNTS:DESERT:260:1:1]
[REGION_COUNTS:FOREST:1040:3:3]
[REGION_COUNTS:MOUNTAINS:2080:2:2]
[REGION_COUNTS:OCEAN:2080:1:1]
[REGION_COUNTS:GLACIER:0:0:0]
[REGION_COUNTS:TUNDRA:0:0:0]
[REGION_COUNTS:GRASSLAND:2080:3:3]
[REGION_COUNTS:HILLS:2080:3:3]
[EROSION_CYCLE_COUNT:250]
[RIVER_MINS:100:100]
[PERIODICALLY_ERODE_EXTREMES:1]
[OROGRAPHIC_PRECIPITATION:1]
[SUBREGION_MAX:2750]
[CAVE_MIN_SIZE:5]
[CAVE_MAX_SIZE:25]
[MOUNTAIN_CAVE_MIN:25]
[NON_MOUNTAIN_CAVE_MIN:50]
[ALL_CAVES_VISIBLE:0]
[SHOW_EMBARK_RIVER:2]
[SHOW_EMBARK_POOL:2]
[SHOW_EMBARK_M_POOL:2]
[SHOW_EMBARK_M_PIPE:2]
[SHOW_EMBARK_CHASM:2]
[SHOW_EMBARK_PIT:2]
[SHOW_EMBARK_OTHER:2]
[SHOW_EMBARK_TUNNEL:2]
[TOTAL_CIV_NUMBER:20]
[TOTAL_CIV_POPULATION:20000]
[PLAYABLE_CIVILIZATION_REQUIRED:1]
[ELEVATION_RANGES:2080:4160:2080]
[RAIN_RANGES:2080:4160:2080]
[DRAINAGE_RANGES:2080:4160:2080]
[SAVAGERY_RANGES:2080:4160:2080]
[VOLCANISM_RANGES:2080:4160:2080]
And now I can only move that big X to 3 tiles from the right and 2 tiles from the left and bottom and all the way up so its like this
MMOOOOMMM
MMOOOOMMM
MMOOOOMMM
MMOOOOMMM
MMOOOOMMM
MMMMMMMMM
MMMMMMMMM
Sorry again if it's hard to understand. Anyway I'll probably just restart at the same area because I don't know if I can get to all the stuff that I wanted to because I usually just increase my starting area to include everything I want (magma, rivers, etc...) plus 1 tile so I can get to it all and a little beyond it if I want.