I have 25 yr old SEER 10 GE heat pump. Heat exchanger is in excellent condition. If I go to R410 I need to replace lines and both units. If I stay R22 I can do only condenser. If I go to SEER 16 do I get any benefit or do I just do the SEER 13. As I get it, either way I only need to replace condenser unit and recharge - no valve or line or other replacements are required although I will have less efficiency than all new system.

1- Its time to go 410A. R22 is on its way out.
2- You will need to replace the evaporator if you go with 410A and if you bought a R22 13 seer condenser with out replacing the evap coil you will not have a true 13 seer system.
3- Go with the highest SEER system you can afford.

There's a lot of ac companies (most) that won`t warranty a new condenser 13 seer or more if you don`t install a matching evaporator coil,, would hate for someone to find this out down the road when its to late

Coil needs to match new outside unit to get the rated efficiency.

R-22 will be around for 20 to 25 years but the equipment will not be manufactured after this year. R-410 works well but is not as forgiving as R-22 in regards to moisture problems with the oil. Different oil is used with the different refrigerants.
R-22 operates on less pressure than R-410. R-410's equipment is designed to operate under the higher pressure.

What's the problem with your existing system?

Existing system can not always start - has a booster capacitor. Especially problematic on 100+ deays - appears that compressor can not maintain pressures. I do recognize a mix of SEER would not give a true or full efficiency gain but it would seem if either major elkement were more efficient that benefit would still be fully obtained. It would seem that pressures etc for R22 would be same regardless of SEER and thus the cheapest alternative even though not as cheap to operate. Lots of discussion threads disparage R410. Maybe this is not a good time to be an early adopter, especially as it means complete new system incluidng freon plumbing between units. I have not done much price research but it sound like $2k vs $10K. That takes a lot of efficiency to make up!

Existing system can not always start - has a booster capacitor. Especially problematic on 100+ deays - appears that compressor can not maintain pressures. I do recognize a mix of SEER would not give a true or full efficiency gain but it would seem if either major elkement were more efficient that benefit would still be fully obtained. It would seem that pressures etc for R22 would be same regardless of SEER and thus the cheapest alternative even though not as cheap to operate. Lots of discussion threads disparage R410. Maybe this is not a good time to be an early adopter, especially as it means complete new system incluidng freon plumbing between units. I have not done much price research but it sound like $2k vs $10K. That takes a lot of efficiency to make up!

It sounds as if you are trying to get away with not changing the inside coil ( you refer to it as a heat exchanger and technically you are correct)
When 10 seer transitioned to 13 seer originally, they just upsized the inside coil. Any change made should start with the inside coil unless you can find a 10 seer outside unit r22. Those units built 25 years ago were awesome. 10 seer r22 equipment was very forgiving.
Heat pumps have come a long way since then. I do know many service companies that are cleaning linesets instead of replacing them. There are chemical purges available. Make sure that the new equipment matches the lineset size or it will have to be re[laced anyway. Most of the r410 uses larger linesets unless you are just lucky.
Make sure to take advantage of the tax credit and buy the best Seer rating that you can afford. I do agree that it is hard to rationalize paying thousands more in the short term but that is not the issue. You need your equipment to match for proper performance to protect the compressor.

Just one(1) question, where can you still buy a r-22 system, I`ve check many resourses and I can`t seem to find any,since they stopped building them in January of 2010

eBay check there