420A HELP! P0731 fault

[G420a]

I have a 1998 eclipse gs automatic i got it a year ago, it's my daily not looking for performance I just use it to go out to the stores, work etc..
Not too long ago on my way home I got a p0731 code gear 1 incorrect ratio and p0715 input speed sensor circuit. Well I changed that sensor already but I'm having trouble with the p0731. I was recommended by someone to change the transmission fluid but I was also told not to or it'll damage the transmission more.
The car was garage kept when I got it last year it still had tags from 2018, it's currently parked and haven't drove it ever since. Back to the point about the fluid, the previous owner didn't change it. So I don't know what to do, I saw that someone changed their input speed sensor as well and it fixed it for them too but I still need help please!!

 

[BLACK'98DSM]

It is recommended to not change the transmission fluid if it has been too long since the last time it was changed, or never changed. But this only applies on high mileage applications. This is because the trash in the fluid actually prevents the trans from slipping. How many miles are on the car? It might still be safe to change it.

 

[motomattx]

Actually the reason for the old urban legend about not changing old fluid was because the new fresh fluid has seal conditioners in it that will soften old cracked oil seals and possibly cause a leak, you can change it if you want to, are you sure its even full?

 

[G420a]

It is recommended to not change the transmission fluid if it has been too long since the last time it was changed, or never changed. But this only applies on high mileage applications. This is because the trash in the fluid actually prevents the trans from slipping. How many miles are on the car? It might still be safe to change it.

it currently has 175k miles, I got it last year and ever since I only put 5k in it.