to determine distance between two positions use

vec_dist

to remove entities use

ent_remove or ptr_remove

to create entities use

ent_create


to understand these functions use

the manual or do the lite-c walkthroughs and examples ,there is also aum magazines .

your code would be something along the lines of

//loop start
....
//if new entity not created yet then proceed
if (vec_dist (ent1.x,ent2.x) == 100);
{
//remove ent2 here
//create new animated entity here
//switch on a variable to make sure you create the //entity only once
}
......
wait (1);
//loop end