printf("Dump File-name：");  scanf("%s",filename1);
    printf("Dest.File-name：");  scanf("%s",filename2);

#include <stdio.h>
#include <string.h>

#define MAX_NAME    32

int main(void)
{
    char filename[MAX_NAME];
    char cpfile[MAX_NAME];
    char ex[MAX_NAME];
    char *p_c;

    printf("input name ==>");
    scanf("%s", filename);

    strcpy(cpfile, filename);
    p_c = strrchr(cpfile, '.');
    if (p_c != NULL) {
        strcpy(ex, (p_c + 1));
        *p_c = '\0';
    }

    if (0 == strcmp(ex, "txt"))
        strcat(cpfile, ".doc");
    else if (0 == strcmp(ex, "doc"))
        strcat(cpfile, ".txt");
    else
        strcat(cpfile, ".err");

    printf("%s\n%s\n", filename, cpfile);

    return 0;
}

で。
char fname[260], *cp;

scanf("%s", fname);
cp = strrchr(fname, '.');
if(cp = !strcmp(cp + 1, "txt")  ? cp : NULL)
	strcpy(cp, ".doc");

> cp = strrchr(fname, '.');
> if(cp = !strcmp(cp + 1, "txt")  ? cp : NULL)

if(cp && !strcmp(cp + 1, "txt"))

    char basename[128], txtname[128], docname[128];

    ... /* basename ← 拡張子を取り除いた名前。fgets()なりscanf()なりで。 */
    sprintf(txtname, "%s.txt", basename);
    sprintf(docname, "%s.doc", basename);