G.I.'s Will Gradually Leave Korea DMZ to Cut War Risk
United States and South Korea agree to reposition American troops far away from demilitarized zone that separates North and South Korea to make them less vulnerable to attack by North; 14,000 American troops are considered strategic 'tripwire' on Korean peninsula, ensuring that American troops would be drawn into any war started by North; redeployment, which will occur over period of years, is negotiated at time of high tensions beetween US and North Korea over North's suspected development of nuclear weapons; it comes after months of strained relations between US and South Korea, where Pres Roh Moo Hyun won election in December amid large-scale protest movement against American military presence in country; most of 37,000 American troops in South Korea are based in northern tier of country, in and around Seoul; some South Korean officials feared that repositioning of troops would leave US free to make pre-emptive attack against North Korea's nuclear sites without fear of retaliation against American soldiers; their fears are eased when US pledges that some American troops will remain presence in northern part of country, near demilitarized zone; photo (M)