Let´s take some very simple example (since don´t see your scene file) with just plane object...
1, Added simple plane object, created material with checkerboard for better control of streching
2, Converted to polygonal object. Now you can see UVW tag was created (this is most simple example, so you could see polygonal mesh and UV mesh looks equaly)
From now material use this UVW "map" for applying textures/materials.
3, Created polyselection/stored and applyied material on selection. Now you could see no stretching materials since was not modified polygonal/UV mesh. Material is applyied on selection nicely without stretching
3a, Now it you change polygonal mesh (moving points to the side), material became stretching. It´s because was changed polygonal mesh only and UV mesh hold original state. Since mesh polygons are enlarged and its uv polygons are not changed, material/texture representation in viewport is stretched...
(There´re 2 options now. One is "adjust" uv mesh to polygonal (enlarge to same ratio/relative dimensions, this is reason why is used checkerboard to adjusting this) or "Reset" UV map (in real this means creating new UVW tag)
4, So, create copy of plane, remove material and uvw tag from it. Now you could see condition for creating uv map is changed since polygonal mesh is different as at startup.
5, Apply material on object with parametric projection (cubic in my example). Since currently object don´t have uvw tag, applying uvw projection doesn´t make sense.You could see texture is applyied correctly.
6, Now its time to create new uv map. Select texture tag and object and from menu select Generate UVW coordinates command
7, As you could see, newly created uv mesh is not fitted into 0-1/0-1 coordinates and needs to be repaired with fit UV to canvas command
8, Now when is created/adjusted uv mesh, you could change projection from parametric to uvw mapping (if it was not done automatically while creating new uvw coordinates. Texture/material now looks identically as on previous plane even polygonal mesh was "adjusted" by moving points.
9, Now with applying polyselection to material restriction you can see no stretching in result. Since now is taken into account this newly created uv mesh.
Now when finished adjusting polygonal mesh and created new uv´s , material/polyselection could be removed/applyied again without stretching
(ofcourse, moving some points again "brake" current "connection" between polygons and uv´s and material again became stretched...)
Maybe this you ment like "reset".
If you don´t want re-create uvw tag again and again after changes on polygonal mesh, you could do it manually by adjusting uv mesh in uv editor by an eye for some sufficient result which is enough for you...