Kilayim, Chapter 2, Mishnah 4. Our Mishnah discusses a sown field or a vineyard whose owner wishes to convert it - from a vineyard into a sown field, or from a sown field into a vineyard.
From a sown field to a vineyard:
"Zarua v'nimlach linto'a" - a field in which grain or vegetables are growing, and its owner reconsiders and decides to plant trees in it, that is, to turn it into a vineyard. Here the Mishnah establishes two rules:
"Lo yomar: eta v'achar kach ehefoch" - he may not plant the vineyard first and uproot the seeds afterward, and he may not rely on doing so.
"Ela hofech v'achar kach note'a" - he must first uproot the seeds, as was explained in the previous Mishnah, that from the moment the seed begins to send out roots it may be uprooted, and only afterward may he plant the trees.
From a vineyard to a sown field:
"Netu'ah v'nimlach lizro'a" - a vineyard that is planted and growing, and its owner reconsiders and decides to turn it into a vegetable field or a grain field. Here too there are two rules:
"Lo yomar: ezra v'achar kach ashrish" - he may not sow first and remove the roots of the vineyard afterward.
"Ela mashrish v'achar kach zore'a" - he must first uproot the vineyard and remove its roots, and only afterward sow.
An additional option: "Gomemo":
The Mishnah adds another course of action for one who has a vineyard and wishes to sow vegetable or grain seeds in it: "Gomemo ad pachos mitefach, zore'a, v'achar kach mashrish" - he trims the vine down until it is less than a tefach high, and then he may sow, and afterward remove the roots.
The reason for this: once he has cut the vine down and made it so low, less than a tefach, it is considered as though it does not exist, because of its appearance. Even so, halachically he is still obligated to remove the roots entirely, but he is permitted to wait and perform the removal of the roots after the sowing.
In summary: This Mishnah establishes that the conversion between a sown field and a vineyard is always done in the order of removing the old before planting or sowing the new: what is sown - "hofech v'achar kach note'a", and what is planted - "mashrish v'achar kach zore'a". With a vineyard, however, there is an additional course: trimming the vine down to less than a tefach, so that because of its appearance it is considered as though it does not exist, and then one may sow and remove the roots afterward.