Unstable Frontier — ruling 1

You choose a basic land type as the ability resolves.

Unstable Frontier — ruling 2

The affected land loses its existing land types and any abilities printed on it. It becomes the chosen basic land type and now has the ability to tap to add one mana of the appropriate color. Unstable Frontier's ability doesn't change the affected land's name or whether it's legendary or basic.

Unstable Frontier : règles, fonctionnement et interactions (MTG)

Unstable Frontier is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

: Add . : Target land you control becomes the basic land type of your choice until end of turn.

How it works : Unstable Frontier

Original English text — Wizards of the Coast / Scryfall

#1February 1, 2009· Wizards of the Coast
You choose a basic land type as the ability resolves.
No other card is named in this ruling.
#2February 1, 2009· Wizards of the Coast
The affected land loses its existing land types and any abilities printed on it. It becomes the chosen basic land type and now has the ability to tap to add one mana of the appropriate color. Unstable Frontier's ability doesn't change the affected land's name or whether it's legendary or basic.
No other card is named in this ruling.